Response: There was a time when the creation did not exist. Did God fundamentally change when He became a Creator? There was a time when the nation of Israel was enslaved by Egyptians. Did God fundamentally change when He became their Deliverer?
According to Trinitarian doctrine each person of ‘The Trinity’ enjoys all the attributes of Deity, which would include immutability. This would mean that from eternity past the alleged second person of ‘The Trinity’ could not change. How, then, could this second person permanently take on human flesh two thousand years ago? Would this not have been a fundamental change? Certainly, it would. Therefore, God’s attribute of immutability means Jesus could not have been God.
NOTE : Some think that Hebrews 13:8 which says, “Jesus Christ is the same yesterday, and today, and forever” affirms his immutability, thus allowing him to be God. However, the verse says “yesterday,” not ‘eternity past.’ Since his resurrection Jesus is unchanging. Contrast with Psalms 90:2 where it says of God, “…from everlasting to everlasting You are God.” (Notice, too, that the pronoun You here is singular, not plural.)