Athanasius was the bishop of Alexandria in Egypt for 45 years in the 4th century AD. The church at that time came under attack from false teachers including Arius, who taught that Jesus was created by, but not equal to, God the Father. Although banished temporarily after the Nicene Council, the Arians gained influence in Constantine's court and were allowed back into the church despite the vigorous protests of Athanasius.
On the Incarnation of Our Lord lays out his understanding of how the Incarnation is central not only to Christian faith but also for the entire course of history to a fellow Christian named Marcarius. He discusses the Trinity, Creation, the fall of Man, the condition of man after the fall, the incarnation, and the redemptive work of Christ while refuting objections to each of these points that were raised during his time. He spends the last few chapters specifically refuting first the objections of the Jews, and then the Gentiles ending with an exhortation to Marcarius to copy of the lives of the saints in action and to study the Scriptures himself in order to have a right understanding of them.
While I found the text challenging, it was far more clear and concise than more modern commentaries I have read on the subject. It is an excellent glimpse at basic Christian theology from an excellent teacher that I would recommend for every Christian to read.