An often cited Theophany of Christ occurs in the visit of the "three men" to Abraham and Sarah in Genesis 18:1-16: "Then God appeared to him at the oak of Mamre" (v. 1). Though three men are there, Abraham addresses them in the singular, "Lord." He responds in the singular (vv. 9-15). As Saint Ephraim the Syrian says, "Therefore the Lord ... now appeared to Abraham clearly in one of the three." The three are generally considered to be Christ the Lord, along with two attending angels.
At Genesis 32:25-31, Christ is the "man" who wrestles with Jacob, after which Jacob says, "I saw God face to face" (v. 30). Saint Cyril of Jerusalem asks the Jews concerning these Theophanies to, Abraham and Jacob, "What strange thing do we announce in saying that God was made Man, when you yourselves say that Abraham received the Lord as a guest? What strange thing do we announce, when Jacob says, 'For I have seen God face to face, and my life is preserved'? The Lord, who ate with Abraham, also ate with us."
At times Christ appears as "the Angel of the Lord" or "the Angel of God." At Exodus 3:1-4:17, "the Angel of the Lord" appears to Moses in the burning bush and identifies Himself as the God of Abraham, Isaac, and Jacob (Exodus 3:6, 15, 16; 4:5). He also says that His name is "I AM WHO AM" (Exodus 3:14), which in Greek is represented by the three letters placed around Christ's head in the holy icons. Saint Ambrose of Milan observes, "Christ therefore is, and always is; for He who is, always is. And Christ always is, of whom Moses says, 'He that is has sent me.'"
When God the Son became incarnate, this can be called an everlasting Theophany For having assumed human nature, Christ not only manifests God to the world during His earthly life (John 1:14; 14:9; Colossians 2:9; 1 John 1:1-3), but He ascends into heaven in the same glorified flesh in which He will return at His Second Coming (Acts 1:9-11).
At the baptism of Christ (Matthew 3:13-17), a further Theophany occurs, as all three Persons of the Holy Trinity are made known: the Father in the voice from heaven, the Holy Spirit in the form of a dove, and the Incarnate Son. Hence, the feast day commemorating this event is known as Holy Theophany. On this day the Church sings, "When Thou, O Lord, wast baptized in the Jordan, the worship of the Trinity was made manifest."
Taken from The Orthodox Study Bible: Ancient Christianity Speaks to Today's World, page 1242, with edits.