Christianity can be seen as an explanatory hypothesis to account for certain phenomena we observe in the world: the origin of the universe, the design of the universe, and the universality of morality. The explanations that Christianity provides to this empirical data provide a cumulative case for the rationality of Christianity, and in fact, the superiority of Christianity to other belief systems. Monotheistic religions (namely Judaism, Islam, and Christianity) enjoy a special rational advantage over other kinds of religious beliefs in their explanatory power, but, as will be shown, Christianity is the most rationally satisfying. Three criteria can be applied to developing the positive case for Christianity and simultaneously the negative case against other religious belief claims: logical consistency, empirical adequacy, and existential viability.
Logical consistency seems to be more "cut and dry" than the other two criteria. Logic is a natural function of human minds and is universally practiced. In fact, the universality of logic itself needs to be explained in a religious worldview. Many religious beliefs can be dismissed at the outset because they fail this first criterion, for example eastern religions that deny rationality and logic. They not only cannot account for logic, they fail the test of rationality since they hold contradictory beliefs.
Christianity is sometimes accused of being illogical, but on closer inspection is rigorously logical. The doctrine of the Trinity is often dismissed as illogical, but is only done so because it is misunderstood or mis-defined (sometimes by ill-equipped Christians). The charge that the Trinity entails a contradiction fails from a category fallacy because the doctrine is sometimes defined as "three gods and one god," or more simply "three in one." The doctrine of the Trinity was carefully and meticulously formulated by the early church with great attention to the laws of logic and metaphysical categories. In fact, much of the first 1500 years of the history of Christianity was marked by great intellects who recognized that, if God created the world, then rationality was one of the features of the creation and it must be brought to bear on the development of doctrine.
Monotheistic religious beliefs also explain the existence of rationality and logic. The laws of logic seem to have a different feature than the laws of nature. The laws of nature are a posteriori inductive observations about how the world functions. There does not seem to be an necessity to them though they are regular, nor is the violation of them in a supernatural act really the same as if the laws of logic were violated. In fact, it is impossible to imagine a real world scenario where the laws of logic could be violated. The laws of logic are discovered a priori, they are features of the mind that bear on our thinking before it even begins. There is an incumbency to them that we cannot escape. The best explanation for this seems to be a rational Designer of the universe Whose own rationality is reflected in the operation of our minds.
Morality is the same kind of case. There is an advance incumbency to morality that is not just an a posteriori observation about how humans tend to function. There is an obligation feature to morality that cannot be reduced or explained away by naturalistic theories. Other religions than Christianity, and even atheists, can offer explanations for morality, but none seem to capture our true experience of morality as monotheistic religions do. Morality cannot be explained adequately in functional or reductionistic terms because they omit a central feature of morality, namely the obligation and quality of morality. Naturalist explanations for morality have a much less satisfying explanation of the grounding requirement of obligation. Obligation seems best understood in terms of minds and persons. A "thick" naturalism, as Plato’s, may be able to account for non-physical features of the universe, such as morality, but the obligation feature is still troublesome. Obligation seems to beg for another person to be obligated to. Morality may be a natural feature like health, but it is normative in a different kind of way than mere observation of proper functioning. The obligation is best understood in terms of persons, which narrows the field of religions that can account for this feature.
Empirical adequacy is a second criterion that should guide the evaluation of possible religious explanations. Science is a deprived discipline without the possibility of theistic explanations. There are good scientific and philosophical reasons to believe that the universe had a beginning and that that cause was personal. Clearly this would rule out natural explanations for the origin of the universe. This also rules out some religious beliefs that maintain that the universe is infinite. The personal cause must be adequate for the origination of the universe, and the monotheistic religions describe this kind of God. The God of monotheism is the best explanation for certain features of design we encounter in the universe. Rationality and logic, morality, regular laws of nature all seems to suggest a Designer of a certain kind: one who reflects these kinds of features.
Existential viability is the final criterion and the most subjective. This criterion is not about the most convenient or appealing religion to live out. It is the most deeply satisfying in a way that is incongruent with the modern notion if satisfaction or happiness; it is a teleological notion. It deals with deep, fundamental issues human beings question. Christianity best answers the challenge of evil in the world. Not only does Christianity explain the origin and redeeming purpose of evil, it gives it a rational basis in that it is the deviation of the good that was innate in creation as God made it. Thus far, Islam and Judaism fare well in their explanation. However, Christianity offers a rational and satisfying solution to evil with the final judgment and the offer of forgiveness for the evil we perpetrate. Christianity does not deny the reality of evil or make it a necessary part of the universe; it presents it as an aberration that will be corrected on a cosmic and personal level. Eastern religions and New Age variations do not account for the real experience of evil, nor do they offer a satisfying solution.
Monotheistic religious beliefs offer existential satisfaction in offering a relationship with a personal God. The universe is real and our desire for relationship with a Person is real. The goal of Christianity is not to deny the reality of the world or our personal identity, as some religious beliefs offer, but to satisfy our deepest longings.
Much of the evidence thus far is congenial to three monotheistic religions: Judaism, Islam, and Christianity. How can Christianity be demonstrated to be rationally superior to these other viable options? First, Jesus made substantive authority claims that were uniquely affirmed by His acts, including the resurrection, and contemporary testimony. The Bible offers historical evidences that can be verified that do not stop in the Old Testament, but continue in the New Testament and the coming of the Messiah. Jesus grounds His authority in His miraculous works and the resurrection, which are all testified to by eyewitnesses. He is the promised Messiah of the Old Testament and the most superior Prophet sent by God. The New Testament offers fulfillment of Old Testament prophecies and makes a more substantive authority claim than the Koran. Jesus is not only a prophet, but God Himself; and Jesus validates His identity claim with His deeds, including the resurrection. It also offers further information that solves Old Testament problems, such as the hints of the Trinity. The New Testament also offers a New Covenant that is superior to the promises and relationship that Judaism and Islam offer. Christianity is the fulfillment of Judaism and God’s plan in the Old Testament.
Christianity offers the most rational satisfaction of the three criteria. It best incorporates and explains our experience of the world and our own existential experience. Christianity takes logic seriously, and, indeed, grounds logic. Christianity is the most rational of religions, and therefore it can be recommended as superior to all other religious beliefs.