St Athanasius of Alexandria wrote On the Incarnation as the second half of a two-part work, after Against the Heathen. It is short, fifty-seven brief chapters, and written for a reader who wants to understand the faith, not for specialists.
Its question is the one at the heart of the Christian faith: why did God become man? St Athanasius answers that humanity, made in God's image, had turned away from Him and was sliding back toward nothing, into corruption and death. God would not let His creature perish. So the Word, through whom all things were made, took a body from the Virgin, and in that body died and rose, overcoming death for all.
He came, Athanasius writes, to renew the image of God in us, to destroy death, and to make the Father known through the works He did in the body. Near its close he writes the line the Orthodox Church has repeated ever since: "He was made man that we might be made God."
C.S. Lewis wrote the introduction to a twentieth-century English translation, urging readers to put old books alongside new ones. That essay is now known as On the Reading of Old Books, and many readers have come to St Athanasius through it.