I think for the hypostatic union, we know that He was God. We also know that he was born of a human Virgin where he took on flesh and became Man. So these two natures became unified as part of the Incarnation.
Scripture
Ex. 3:14 - In the Old Testament, God reveals His name as “I AM who AM”.
Jn. 8:24,58 - In the New Testament, Jesus says of Himself, “Before Abraham was, I AM”.
Lk. 1:31-32 - The angel of the Lord proclaims Jesus as Son of the Most High.
Mt. 2:2,11;8:2;14:33;28:9;28:17;Lk. 24:52;Jn. 9:38 – Jesus allows Jews to worship Him. Only God can be worshipped.
Mt. 9:2;Mk. 2:5;Lk. 5:20;7:48 – Jesus forgives sins. Only God can forgive sins.
Mt. 4:7;Lk. 4:12 – Jesus tells Satan, “you shall not tempt the Lord your God” in reference to Himself.
Mt. 12:8;Mk. 2:28;Lk. 6:5 - Jesus says that He is “Lord of the Sabbath.” He is the Lawgiver. He is God.
Mt. 26:63-64;Mk. 14:61-62;Lk. 22:70;Jn. 10:30 - Jesus claims to be the Son of God.
Jn. 1:1;Rev. 1:8;22:12-13 - John says the Word and God are one. Jesus Christ is the Word; the Alpha and Omega.
Jn. 20:28 - St. Thomas addresses Jesus as “My Lord and my God!”.
Tradition & Fathers
“We have also as a Physician the Lord our God, Jesus the Christ, the only-begotten Son and Word, before time began, but who afterwards became also man, of Mary the virgin.”
St. Ignatius of Antioch, To the Ephesians (A.D. 110)
“For if you had understood what has been written by the prophets, you would not have denied that He was God, Son of the only, unbegotten, unutterable God.”
St. Justin Martyr, Dialogue with Trypho (A.D. 155)
*“But nothing exists, the cause of whose existence is not supplied by God. Nothing, then, is hated by God, nor yet by the Word. For both are one–that is, God.” *
St. Clement of Alexandria, The Instructor (A.D. 202)