C.S. Lewis wrote an introduction to an English translation of Saint Athanasius’ On the Incarnation which is on-line here.
In his introduction Lewis made the following observation: “I myself was first led into reading the Christian classics, almost accidentally, as a result of my English studies. Some, such as Hooker, Herbert, Traherne, Taylor and Bunyan, I read because they are themselves great English writers; others, such as Boethius, St. Augustine, Thomas Aquinas and Dante, because they were “influences.” George Macdonald I had found for myself at the age of sixteen and never wavered in my allegiance, though I tried for a long time to ignore his Christianity. They are, you will note, a mixed bag, representative of many Churches, climates and ages. And that brings me to yet another reason for reading them. The divisions of Christendom are undeniable and are by some of these writers most fiercely expressed. But if any man is tempted to think—as one might be tempted who read only contemporaries—that “Christianity” is a word of so many meanings that it means nothing at all, he can learn beyond all doubt, by stepping out of his own century, that this is not so. Measured against the ages “mere Christianity” turns out to be no insipid interdenominational transparency, but something positive, self-consistent, and inexhaustible. I know it, indeed, to my cost. In the days when I still hated Christianity, I learned to recognise, like some all too familiar smell, that almost unvarying something which met me, now in Puritan Bunyan, now in Anglican Hooker, now in Thomist Dante. It was there (honeyed and floral) in Francois de Sales; it was there (grave and homely) in Spenser and Walton; it was there (grim but manful) in Pascal and Johnson; there again, with a mild, frightening, Paradisial flavour, in Vaughan and Boehme and Traherne. In the urban sobriety of the eighteenth century one was not safe—Law and Butler were two lions in the path. The supposed “Paganism” of the Elizabethans could not keep it out; it lay in wait where a man might have supposed himself safest, in the very centre of The Faerie Queene and the Arcadia. It was, of course, varied; and yet—after all—so unmistakably the same; recognisable, not to be evaded, the odour which is death to us until we allow it to become life:
an air that kills
From yon far country blows.
We are all rightly distressed, and ashamed also, at the divisions of Christendom. But those who have always lived within the Christian fold may be too easily dispirited by them. They are bad, but such people do not know what it looks like from without. Seen from there, what is left intact despite all the divisions, still appears (as it truly is) an immensely formidable unity. I know, for I saw it; and well our enemies know it. That unity any of us can find by going out of his own age. It is not enough, but it is more than you had thought till then. Once you are well soaked in it, if you then venture to speak, you will have an amusing experience. You will be thought a Papist when you are actually reproducing Bunyan, a Pantheist when you are quoting Aquinas, and so forth. For you have now got on to the great level viaduct which crosses the ages and which looks so high from the valleys, so low from the mountains, so narrow compared with the swamps, and so broad compared with the sheep-tracks.”
I think there is much validity in what Lewis wrote. From a Catholic perspective, a non-Catholic Christian writer writing on Christianity will usually only have part of the truth, and what portion that will be will vary depending upon the writer’s denomination. However, from the perspective of a non-believer the unities of belief between all Christians must seem very large indeed. This is not an argument for indifferentism between Catholics and non-Catholics, but I do think it is something for Catholics engaged in evangelization to ponder when explaining the Faith to a non-believer.