It is a shame that more people haven't read this book; after the New Testament, Athanasius' De Incarnatione Verbi Dei (On the Incarnation of the Word) is the most important synthesis of Christian thought up through the 4th century and has remained one of the most foundational of all Christian texts ever written. All later Christian thought on the sacraments and artwork (particularly icons) would eventually be an extension of the Christian teaching on the Incarnation; this work, then, not only shapes the basis of later Christian thought, but also determines its trajectory.
This is a simple work. Some of this is due to the work of the translator, breaking up the work into short sections and translating it into contemporary English without sacrificing its content; the majority of it has to do, however, with Athansius' own desire: to communicate simply the profound message of God-become-man. C. S. Lewis contributes a wonderful introduction, noting correctly that we would all do better to "read the old books", such as this one.
In short, Athanasius writes that "God became man so that man might become god". If taken out of its context, such a quote could easily be misinterpreted; it should be understood, however, in this way: by God's taking on a human body, the human body has been brought up into the very life of God. Rather than denigrating physical, created matter, the Incarnation vindicates its being created. The body then, is now understood as the site of the most profound of meanings: its being given life now and, at a future time, being given life again.
Understandings of the Incarnation as being purely juridical, with effects relegated to an ethereal world of purely legal justification, find no place here. Athanasius also does not focus upon the death of Christ or his sufferings as ends in and of themselves. Rather, the Incarnation is victory over death - death *not* being a curse but, instead, the natural result of man's turning away from God (the hermeneutic that Athanasius provides for understanding the Apostle Paul's writings is both fascinating and beautiful). The Incarnation opens us up to union with God, which is most perfectly demonstrated in the union of Christ to God the Father: their wills in perfect communion with each other, in and through love.
In this work, Athanasius strikes a perfect balance between the profound and the simple that is not often found in theological writings. We do well, as Lewis notes, to read the old books.