Alvin Plantinga is a philosopher and theologian whose work significantly reshaped contemporary Christian philosophy and apologetics. Before his work during the 20^th century, the philosophical discussion was dominated by classical evidentialism, a position rooted in Enlightenment figures like John Locke and David Hume. This view demanded that for any belief to be rational, it must be “proportioned to the evidence”, meaning material observation. During the 20th century, this stance was intensified by logical positivism, championed by figures like A.J. Ayer. Positivism’s verification principle asserted that any statement not empirically verifiable by observation or true by definition (like “all bachelors are unmarried”) was literally meaningless. This rendered theological statements like “God exists” not just unproven but unprovable and nonsensical. This philosophical position created an impossible and unreasonable burden of proof for the theist, but ironically, also for the position of logical positivism itself. Belief in God was considered irrational by default. The apologist’s sole task, by demand of the positivists, was to construct propositional arguments (cosmological, teleological, moral) from supposedly neutral premises, attempting to convince a sceptic that belief in God could be, at minimum, intellectually respectable.

Plantinga challenged this positivist assumption by attacking its root of classical foundationalism. This philosophical position held that all beliefs must be based on other beliefs, except for a privileged “foundation” of beliefs that were considered properly basic. For classical foundationalists, properly basic beliefs were limited to only two categories: those that were self-evident, such as 2+2=4 and those that were incorrigible. This second category referred to direct reports of one’s own immediate conscious experience, beliefs about which one supposedly cannot be mistaken such as ‘I feel pain’ or ‘I seem to see a red patch’. Plantinga’s critique was twofold. First, he argued the criterion was self-defeating. He formulated the classical foundationalist’s criterion as: “A proposition p is properly basic for a person S if and only if p is self-evident to S, or incorrigible for S.” Plantinga then asked if classical foundationalism itself meets its own criteria. It is clearly not self-evident (many rational people have denied it), nor is it incorrigible. It also cannot be based on evidence that meets this standard. Therefore, Plantinga concluded, “classical foundationalism is self-referentially incoherent and cannot be rationally accepted.” By its own standard, it is an unreasonable belief. Second, Plantinga argued the standard was far too narrow, excluding many beliefs that are clearly rational. He argued that rational people accept numerous beliefs as basic, including “memory beliefs… beliefs as to the existence of other persons… [and] perceptual beliefs”. Plantinga’s point was that none of these common beliefs satisfy the classical criteria. Belief in the past is not necessarily self-evident, and it is not incorrigible because our memory can be mistaken. Belief in an external world is not self-evident (its denial is not contradictory) and not incorrigible (we could be hallucinating; the belief is about an external object, not just our internal sensation). Belief in other minds is also not self-evident and is clearly not incorrigible, as we can misread another’s emotions. Furthermore, we do not typically hold these beliefs because we constructed a philosophical argument for them. Plantinga’s deeper point is that these beliefs are not just rational, but presuppositional. They are beliefs we must rely on to perform any meaningful observation or engage in any rational inquiry, including science and philosophy. To even begin verifying a scientific claim, we must presuppose the reliability of their memory (to record the data), the existence of an external world (to conduct the experiment), and the reports of other minds (to collaborate and read data). By excluding these necessary presuppositions from its foundation, classical foundationalism failed to account for the very beliefs that make knowledge possible.

In its place, Plantinga proposed Reformed Epistemology. This view, drawing from Reformed thinkers like John Calvin, argues that belief in God does not require the support of propositional arguments to be rational. Instead, belief in God can be “properly basic”. A basic belief is one not inferred from other beliefs. A belief is properly basic when it is rational and warranted to hold it without such an inferential chain. Plantinga argued that classical foundationalism’s criteria for proper basicality (self-evidence, incorrigibility) were far too restrictive. He contended that humans rationally hold many beliefs in a properly basic way. We do not require arguments to believe in the reality of the past, the existence of an external world, or the presence of other minds. These beliefs are not groundless. They are produced immediately in us by certain experiences (memory, perception, social interaction). We do not infer the tree’s existence from our sense data; we simply perceive the tree. The belief is formed directly.

Plantinga argues that belief in God functions similarly, and he establishes this claim on an epistemological model. Drawing from John Calvin, he posits a sensus divinitatis (sense of the divine). This is not a mere feeling, but a natural, built-in human cognitive faculty designed by God to produce belief in God. To explain how this faculty produces knowledge, Plantinga introduces his concept of “warrant”; the specific quality that turns a true belief into genuine knowledge. More simply, warrant is what makes a belief solid and reliable. It is not just about having evidence (which philosophers call justification), but about the belief being formed in the right way by a cognitive faculty that is functioning properly in the environment for which it was designed, according to a design plan successfully aimed at truth. The sensus divinitatis is such a faculty. In certain circumstances, this faculty is “triggered” and produces a belief in God directly. These triggers are not premises in an argument but rather experiences that occasion the belief: observing the grandeur of a mountain, the intricacy of a flower, experiencing deep moral conviction, or feeling gratitude for human recovery. In these moments, the sensus divinitatis causes the person to form the belief “God exists” or “God is to be praised” immediately, not as the conclusion of a process of reasoning. This has been a point of apologetic discussion. Prominent atheists, such as Richard Dawkins, have acknowledged the sense of “awe” or “wonder” when confronting the complexity of the universe. In his “God Delusion” debate with John Lennox, he freely admitted, “I think that when you consider the beauty of the world, and you wonder how it came to be what it is, you are naturally overwhelmed with a feeling of awe, a feeling of admiration. And you almost feel a desire to worship something. I feel this, and I recognise that other scientists, such as Carl Sagan felt this, Einstein felt it.” Dawkins agrees that the impulse exists but argues it is a misfiring of an evolutionary adaptation or a psychological by-product. He concludes that this feeling, while powerful, is not a reasonable “signpost to a creator” but is simply an emotional response to physics and biology. Plantinga’s model suggests that this shared experience is the sensus divinitatis at work. The triggers are real, and the faculty is functioning as designed when it produces belief in God. It is not a “misfiring” but the very “firing” it was intended for.

This model accounts for the biblical understanding that God has made knowledge of Himself “evident within them” (Rom 1:19). Plantinga’s model also incorporates the noetic (knowledge-related) effects of sin. He argues that sin damages this faculty, making it malfunction. This explains why the belief is not universally produced, why it can be resisted, and why the knowledge of God, while readily available, is often suppressed (Rom 1:18).

This approach shifts the task of apologetics. If belief in God is properly basic and rational on its own, the apologist’s burden is no longer to construct arguments to prove God’s existence from a set of neutral or antagonistic premises. Instead, the task becomes defensive. The burden of proof shifts because belief in God is considered rational until proven otherwise. The apologist’s new role is to respond to “defeaters” (a new belief or argument that attempts to overturn an existing belief). Plantinga identifies two main types. A “rebutting defeater” argues that the original belief is false (e.g., “There is no God because evil exists”). An “undercutting defeater” argues that the belief, even if true, is not held for rational reasons (e.g., “Your belief in God was caused by wish-fulfilment, not a reliable faculty”). A common defeater proposed against Reformed Epistemology itself is the Great Pumpkin objection. The argument is, if belief in God is properly basic, why not belief in anything, like the Great Pumpkin? Plantinga’s response is multifaceted. First, beliefs are properly basic only when produced by a cognitive faculty functioning properly in its intended environment. Humans have faculties for external perception and a sensus divinitatis for perceiving God. No such faculty exists for perceiving a Great Pumpkin. Second, the two beliefs are not analogous in their content. The concept of God is one of a necessary, non-physical, eternal being, which Plantinga calls a “maximally great being”. This is a reasonable candidate for a unique cognitive faculty to detect. The concept of The Great Pumpkin is of a contingent, physical object. It is confined to being a member of a known, physical category and is thus not a plausible object for a unique, sui generis (being of its own kind) faculty. It would simply be perceived by our ordinary senses. Third, this distinction highlights a pseudo-ontological failure in the objection. Because the pumpkin is a contingent object, it can never be maximally great, and cannot be exalted beyond its physical category. Likewise, its belief cannot be truly basic in the same sui generis way, as the object itself cannot be reduced beyond its category. It is always and only a pumpkin, an object within the world, not a candidate for the establishment of the world. It is ontologically confined in a way that the concept of God is not. Plantinga argues that belief in God, by contrast, does not face any successful defeaters. Plantinga’s subsequent work, therefore, focuses on identifying the prominent “defeaters” for Christian belief and presenting a case against them. The focus shifts from building a case for belief from a neutral or antagonistic starting point, but showing that no case against belief successfully undermines its rational standing.

Plantinga’s most famous work regarding these “defeaters” is the Free Will Defence. This argument addressed the problem of evil, an argument most famously formulated by the Greek philosopher Epicurus: “Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?” In the 20th century, this was refined into the logical problem of evil by philosophers like J.L. Mackie. This problem, posed as a formal contradiction, states that the following beliefs cannot all be true

1. God is omnipotent
2. God is omnibenevolent
3. Evil exists.

The sceptic’s unstated assumption is that an omnipotent God could create any logically possible world, and that a world containing truly free creatures who only choose good is logically possible. Plantinga’s solution is a defence adjacent to the issue of suffering, not a direct theodicy. He quite reasonably does not claim to know the actual reason God permitted evil (a theodicy). This is possibly the greatest challenge for any Christian to answer. He only aims to show that points 1), 2), and 3) are not logically contradictory. To do this, he only needs to introduce a fourth proposition that is logically possible, theologically consistent and compatible with 1) and 2), and from which 3) originates. His argument rests on two premises. First, a world containing creatures who are genuinely free is more valuable than a world of automatons. Second, omnipotence does not mean the ability to do the logically impossible. He argues that “making someone freely choose good” is a logical contradiction, like “creating a married bachelor”. God can create free creatures, but He cannot simultaneously cause them to freely choose good. The choice must be their own.

The core of his argument is the concept of “Transworld Depravity”. Plantinga invites his reader to consider it logically possible that every free creature God could have created suffers from this condition: in any possible world God could create in which they are truly free, they would inevitably perform at least one morally wrong action. This does not mean sin is a necessity, as if God determined it or the creature was unfree. It means sin is a contingent inevitability or an unavoidable consequence of creating truly free agents. While any single sinful act is freely chosen and could have been otherwise, the fact that some sin would occur was a probability concerning the creatures God had made truly free. If Transworld Depravity is even logically possible, the sceptic’s unstated assumption is false. God’s choice was not between:

1. a world with free creatures who only do good and
2. a world with free creatures who do evil.

His choice was between:

1. a world with free creatures who inevitably do evil or
2. a world with no free creatures at all.

Because a world with genuine free will is a great good, God had a sufficient reason to choose 1). This demonstrates that God, evil, and omnipotence are not logically contradictory. Plantinga also extends this defence to cover natural evil such as earthquakes and disease. He argues it is logically possible that natural evil is the result of the free choices of powerful, non-human free agents. He does not claim this is true, only that it is possible. In doing so, he places all forms of evil under the umbrella of free will, addressing the logical problem in its entirety.

Plantinga also directly addressed naturalism, often presented as the only rational alternative. His Evolutionary Argument Against Naturalism (EAAN) targets the conjunction of philosophical naturalism (N: the belief there is no God or supernatural) and unguided evolution (E: the belief that life developed via unguided natural selection). Plantinga argues that if N and E are both true, then our cognitive faculties were selected purely for their survival value, not for their ability to produce true beliefs. Adaptive behaviour is all that matters for survival, and adaptive behaviour can be produced by false beliefs just as easily, if not easier than by true ones. For example, a creature that falsely believes a boulder is a predator and desires to flee will survive, just as a creature that correctly believes it is a boulder and desires to rest in its shade might also survive. Since the content of the belief is irrelevant to the resulting behaviour, evolution would have no interest in selecting for truth. Therefore, Plantinga argues, the probability that our cognitive faculties are reliable (R) given N and E is low or at best impossible to understand or interpret. If a naturalist accepts N and E and understands this, they acquire a powerful “undercutting defeater” for their belief in the reliability of their faculties. This “defeater” does not argue that N and E are false, but that the naturalist has no reason to trust the faculties that produced that belief. This “defeater” is universal, applying to all beliefs produced by those faculties, including the belief in N and E itself. The naturalist is caught in a self-defeating loop. If N and E are true, they have no rational basis for believing N and E are true. Naturalism, therefore, provides its own “defeater” and cannot be reasonably affirmed. Plantinga notes that this argument does not apply to the theist, who has a strong reason to believe their faculties are reliable because they are given by God, a rational being, who created them in His image to be aimed at truth.

Plantinga’s contribution shifted the apologetic conversation in a time of unreasonable scrutiny and dismissal. He added new arguments, but more crucially, he changed the rules of engagement. He moved Christian belief from a position of needing external validation from a secular, antagonistic starting point to one of inherent rationality. By demonstrating the self-contradiction of classical foundationalism and establishing a model of warranted basic belief, he restored intellectual confidence. The apologist could reasonably assert that belief in God is the proper starting point, placing the burden of proof on the sceptic to provide a defeater that is most reasonable. This freed Christian philosophers to pursue constructive projects rather than only reacting to secular critiques. His work demonstrates that Christian belief is not an intellectual liability, but a robust and coherent foundation for knowledge capable of defending itself against all intellectual objections and, as seen in the EAAN, challenging the rationality of its chief alternatives.