That's today's aphorism - an encouragement to think of, to formulate, Christianity as something common-sensical in its mechanisms and causality, yet incredible in its claims.
Incredibility - It is an error to try and 'normalize' Christianity, to claim that it is obvious and no big deal - that being Christian is merely the product of reason and logic and solid history and that one would have to be uninformed, dishonest or crazy not to believe it.
Actually, Christianity is incredible, stretching of credibility - hard to believe because its claims are so extreme and astonishing; and incredible too in the scope and power of its truth when that truth is understood.
And if 'reasonableness' is one extreme to be avoided, so is paradox. Paradox, beloved of a certain type of intellectual (Charles Williams?) is not sophisticated but a failure to understand. Paradox stuns - it fails to bridge the worldly and heavenly, this life and the next - sooner or later paradox leads to despair; therefore it must be shunned.
When we try to explain Christianity to modern people we should be prepared that it will probably sound to them both as simple as a child's fairy tale and as unbelievable as a child's fairy tale.
It is a mistake to soften this impact, or to dress it up with philosophical imprecision and paradox masquerading as complexity, or to try and diffuse the impact of the strangeness and apparent absurdity of Christianity in a world where nothing is finally believed except that nothing is really real.
Because the bottom line is that Christianity is a story - essentially, the story told by the gospels; extended to including our own personal place in the story - which makes it real - and as a story Christianity resists explanation in terms of 'meaning' (or philosophy) - just as a children's fairy tale becomes alien and unrecognisable when its supposed meaning is explained by an anthropologist, folklorist, or psychologist.
As so often, Tolkien got to the nub of it: Christianity is a Fairy Story that is true - it is the true Fairy Story. The implication, which Tolkien himself didn't follow up - but which CS Lewis did - is that Christianity ought to be explained as a Fairy Story, without compromising in the direction of modern notions of plausibility.
The story is told - and then we must each, as individuals, seriously ask God concerning its truth - ask God within us by meditation, ask God the Father in prayer... whatever - but that is how we can and indeed must evaluate the truth of a story.
(And once we know the story is true, then we can - if we need or wish to - spend the rest of its life in understanding just how it is true.)