According to the Bible, God does not change:
I the Lord do not change. So you, the descendants of Jacob, are not destroyed. (Malachi 3:6)
Essentially God’s nature is forever the same, he isn’t changing over time, so for example God’s nature is to always be perfect, God is always all-knowing and so on, he didn’t gain these attributes or part of his nature after.
Now comes the problem though, at least if you believe Jesus is God in the flesh, as this was a clear change of nature. According to Christian theology, or at least Christian theology concerning those who believe in the deity of Jesus. God who is Jesus became man in the flesh, he became a very real human being, he was true blood and flesh, and had the true human nature.
God basically took on a new nature by becoming a man, this is an obvious change, and so we have a contradiction. If God became fully human and took on this new nature, which is a change, this then contradicts Malachi 3:6 and so the theological belief here is in contradiction with the Bible. Now if there is no contradiction, then the next problem is that this theological belief is therefore false or at least it’s not biblical, because the Bible makes it clear that God does not change, yet according to Christian theology he did change, and very much so by becoming a full human being. Either way you have a problem.
Christian apologists try to argue that there is no problem, they claim that God didn’t change his nature, he merely added onto his nature, so when he became a man he didn’t take away anything from his nature, he was still all-knowing, all-powerful, and everything else he always was, he merely adopted a new aspect to himself by taking on the man nature.
This is one of those moments where Christian apologists really do lower your intelligence, with all due respect. Christian apologists are basically saying God did change his nature, but instead they word it differently, instead of saying God did change, they merely say he ‘added’ to his nature, so they’ve said the same thing but in different wording to make it seem that no change has taken place.
If God ‘added’ a new nature to himself, then that it is a clear change. Was God always a man? No. Did God later on become fully human taking on the human nature-attribute? Yes. Therefore you have a clear change in nature, just because he ‘adds’ something onto himself does not mean there’s no change, adding something to yourself is clearly a change.
So either Christian apologists accept the biblical contradiction, that God did change thus contradicting Malachi 3:6, and therefore bringing the entire Bible’s reliability into question. Or they admit that their theology is indeed not biblical as it is in contradiction with the very Bible they supposedly believe, there is no middle or in-between.