In the introduction we have an instance of Lewis's exhorting us to read more of the Old Books, like this one from the 4th century, though we are timid: "The student is half afraid to meet one of the great philosophers face to face. He feels himself inadequate and thinks he will not understand him. But if he only knew, the great man, just because of his greatness, is much more intelligible than his modern commentator."
And Lewis also writes here about devotional vs. doctrinal works, On the Incarnation being one of the latter, that he finds the doctrinal books often "more helpful in devotion" than the expressly devotional ones. I can relate to his description of people who "find that the heart sings unbidden while they are working their way through a tough bit of theology with a pipe in their teeth and a pencil in their hand."
Actually I don't know about the pipe in the teeth, but I always have a pencil in my hand as I lie in bed with my book of theology or literature or whatever. And I marked some passages from St. Athanasius to share in this season when we focus on God With Us.
You must understand why it is that the Word of the Father, so great and so high, has been made manifest in bodily form. He has not assumed a body as proper to his own nature, far from it, for as the Word He is without body. He has been manifested in a human body for this reason only, out of the love and goodness of His Father, for the salvation of us men. We will begin, then, with the creation of the world and with God its Maker, for the first fact that you must grasp is this: the renewal of creation has been wrought by the Self-same Word Who made it in the beginning. There is thus no inconsistency between creation and salvation; for the One Father has employed the same Agent for both works, effecting the salvation of the world through the same Word Who made it in the beginning.