Today’s Feast of the Baptism of the Lord happens to supersede what would be the normal (optional) memorial of St Hilary of Poitiers. It’s a very appropriate overlap.
In the Gospel for this Mass, we hear proclaimed the words of the voice from heaven: “This is my beloved Son…” He is the full and complete Son of the Father: He is the eternal Word, begotten and not created. He is not something brought about in time; He is the eternal Son of the eternal Father. He is “God from God…true God from true God.” He is the “one mightier…[who] will baptize you with the Holy Spirit and fire.”
And yet, there He is in the Jordan River with John, being baptized like we might, had we been there. He is human, full and complete, just as we are; born in time, son of Mary. The seeming contradiction is held together by our Faith’s declaration that He is both. This is crucial for our Faith that in Christ and His sacrifice we have been redeemed, and our sins forgiven (Ephesians 1:7-8) because He is fully both: human and divine (Colossians 1:19-20; John 1:1, 14).
While in the course of the 4th and 5th and 6th centuries some Christians have wanted to deny this unity (for the sake of respecting the One-ness of God), the teaching of the Church that became the orthodox teaching is the full and true unity of God and human in Jesus, whose self-offering was a gift to the One who was sinned against by one who (by taking our humanity) “was made sin for us” (II Corinthians 5:21).
And so we come to Hilary—known as “the Athanasius of the West” because both saints (one from Egypt and one from modern-day France) fought with utter determination for what was the final truth: that Him who was truly God was made truly human in order to be the complete and final act of reconciliation between God and the human race.
So we thank God for the revelation at the River Jordan; we thank God for the struggle for truth that was the life and writing of St Hilary (and St Athanasius); we thank God for the reconciliation we have with Him, through his Son Jesus Christ.