A Mormon friend, in passing, remarked that religion is not rational, so he didn't expect it to make sense. It's a matter of faith, not reason.
You might think, "Right: Mormon. I don't expect rationality, either." Hang on.
He went on to give an example—but the example was not how a human could become a god, or how there could be only one god and many at the same time, or how God can keep changing His mind about things, or how two equally-inspired books could contradict each other. His example was the virgin birth. I said there was nothing irrational about the virgin birth, and the conversation simply moved on elsewhere. (I now wish I'd asked instead of stated; still looking for a do-over.)
But was he right? Is religion irrational?
"Religion," maybe. Christianity, no.
Now, before we stay too focused on my friend's Mormonociousness, I'd add that some Charismatic friends have said the exact same thing. Try to follow out some thinking to its uncomfortable conclusion, and you get a shrug and a dismissal. It doesn't have to make sense. It's faith, man. "A man with an experience is never at the mercy of a man with an argument," I heard a Charismatic church elder say.
Perhaps definitions are part of the problem. There is a world of difference between rational and rationalism. The latter is a philosophy, a worldview that asserts that man can know truth by the use of his unaided reason. The former merely means that something is in accord with reason, it doesn't violate fundamental canons of thinking such as the law of non-contradiction.
Is Christianity rational? Without re-writing van Til, Gordon Clark, Carl Henry and the gang (—as if I could), I'd rather just focus on one generality and two specifics.
First, some who karaoke this tune are actually simply anti-intellectual. Their religion is a Schleiermacheranian mish-mash of feelings and sentimentality; and, lazily, they like it that way. Like Alice's queen, they have "believed as many as six impossible things before breakfast." They can splop! down an absurd statement and, when challenged to try to make any kind of sense of it—let alone Biblical sense—they can loftily murmur that their religion is a matter of the heart, not of the mind.
This is of course to stand Biblical religion on its head (pun noted, but not intended). As soon as you assert anything about God, life, reality, you find yourself in the arena of thought and ideas. Even the assertion that nothing can be asserted about God is an assertion about God, open for analysis, criticism, acceptance or rejection.
This is by the design of God, who crafted us to analyze, understand, exercise dominion (Genesis 1:26-28). Thus He positions the first commandment as "You shall love the Lord your God with all your heart and with all your soul and with all your mind"(Matthew 22:37; cf. Deuteronomy 6:5).
J. Gresham Machen had fought and slain this dragon a half-century earlier -- nor was he the first. The shade of rouge, the odor of the cheap perfume, and the color of the plastic jewels change, but it's the same old whore.
But second, even among Christians who are not anti-intellectual jellyfish, I've met some who very reverently think that some of our beliefs simply are not rational. They're mysterious, they have to be held by faith, not reason.
To this I'd just begin by noting that the opposite of faith is not reason; it is sight (cf. 2 Corinthians 5:7).
But are some of our faith-tenets irrational? Two that I hear cited specifically are the Trinity, and the Virgin Birth.
The second example is just plain silly. I have never understood how this can be an issue to anyone who grants the premise of a God who created everything out of nothing. It's like saying, "Everything out of nothing? Sure! But make an existing egg alive without a sperm? No way!" Canons of rational thought are not even stretched, let alone violated, by the fact of the Creator and Ruler thus operating within His creation.
How about the Trinity? Surely the doctrine that God is three and one is not rational?
When I informally debated a Jesus-only heretic on the radio once, he described the Trinity as the belief that "God is three people and one person at the same time." That belief is irrational; if that were what the doctrine of the Trinity meant, I would agree with him. God is not one in one way, and three in the same way.
Yes, the Trinity, stated that way, is irrational. That statement is also irrelevant. Because Biblically-instructed Christians do not believe this.
(By the way, this is a classical straw man argument. You'll meet it in every anti-Trinitarian cultist or heretic. The procedure is as old as dirt: mis-state, then refute the mis-statement, then declare victory. This is yet another reason why it is so vital that we know what we believe better than those whom we seek to evangelize.)
The Trinity is the Biblical teaching that there is but one God (Deuteronomy 6:4), and that this one God is Father (2 Peter 1:17), Son (John 1:1), and Spirit (Acts 5:3-4). The simplest way I have been able to understand and express the truth is that God is one in one way, and three in another. Or, we could say that God is one "what" (i.e. one as to His essence), and three "who's" (i.e. three as to His persons).
Now, do we understand the Trinity exhaustively? Of course not! How exactly does God manage being what He is? We don't really need to know, since we'll never need to be God. Nor should the finite expect to understand the infinite exhaustively. It is as C. S. Lewis says:
If Christianity was something we were making up, of course we could make it easier. But it is not. We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with Fact. Of course anyone can be simple if he has no facts to bother about. (Mere Christianity [Macmillan: 1960], p. 145.)But we know enough to love Him, to worship Him, and to discern truth from error. And we know enough to know that there is nothing irrational about the doctrine.
Is Christianity rational? I daresay it's the only worldview, ultimately, that is.
Put another way: if it isn't rational, it isn't Christianity.