During the fourth century, controversy raged in the church regarding the nature of Jesus christ. On one side were the Arians, led by the Bishop Arius, who argued that Father, Son, and Holy Spirit were materially separate from one another. They believed that Jesus had been created out of "non-existence" and thus was not on the same level of dignity as God the Father.
In response, Athanasius, Bishop of Alexandria, wrote On the Incarnation, a staunch defense of the full divinity and full humanity of Christ. In simple language and with Scripture as a guide, he argued for the eternal nature of the Trinity and that Jesus Christ is not a creation of God the Father but has existed from the very beginning. Athanasius celebrates the redeeming work that came forth through the God-man, Jesus Christ, and His eternal existence and essential unity with the Father.
Ultimately, Athanasius was exiled five times by four different Roman emperors due to his defense of the Trinity, but he remained faithful to his beliefs. On the Incarnation is often included on his lists of books every Christian should read.