St. Cyril of Jerusalem: The Triune God

“IF you have sincere piety, the Holy Ghost comes down on you also, and a Father’s voice sounds over you from high—not, ‘This is My Son’ (Mt. 3:17), but, ‘This has now been made My Son’; for the ‘is’ belongs to Him alone, because ‘In the beginning was the Word, and the Word was with God, and the Word was God’ (Jn. 1:1). To Him Belongs the ‘is’ since He is always the Son of God; but to you ‘has now been made’: since you have not the sonship by nature, but you receive it by adoption.”

~St. Cyril of Jerusalem:  Catecheses, No. 4:4.

Baptism of Christ, by Domenico Ghirlandaio.
Fresco, c. 1473; Sant'Andrea a Brozzi, San Donnino.