Many scholars date Athanasius’ greatest work defending the Incarnation of Word of God to right after the Council of Nicaea. (You can read, and should read, the entire work.) Opening the letter, Athanasius writes,
In our former book we dealt fully enough with a few of the chief points about the heathen worship of idols, and how those false fears originally arose. We also, by God’s grace, briefly indicated that the Word of the Father is Himself divine, that all things that are owe their being to His will and power, and that it is through Him that the Father gives order to creation, by Him that all things are moved, and through Him that they receive their being. Now, Macarius, true lover of Christ, we must take a step further in the faith of our holy religion, and consider also the Word’s becoming Man and His divine Appearing in our midst. (1.)
Although most scholars date it this letter to this time, it may be that Athanasius was writing at a much later date than 326-328, perhaps as late at 337, the year in which a presbyter named Macarius and several deacons traveled as deputies from the Eusebian Party to Rome to present the East’s case against Athanasius and Marcellus. (See Apol c. Arianos 22-24). It is possible that this letter was the response to Maracius which Athanasius sent, ultimately leading Macarius to feign illness and excuse himself from the case against Athanasius.
Could this treatise then be the defense offered by Athanasius in 337?