If God is immutable, how can he “become” something he was not (flesh)? Orthodox theology answers: The person of the Son assumes a human nature in time, but the divine nature remains unchanged. It is a relational change, not an essential one.
"He became what we are, that we might become what He is." Incarnation
"In the beginning was the Word, and the Word was with God, and the Word was God... And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth." (John 1:1, 14, ESV) If God is immutable, how can he “become”