God is perfect and He is all powerful. Both Muslims and Christians accept that this is true. The ontological term for this is that God is "immutable" or absolute in His perfect nature. In the Christian faith, God's incarnation is seen as a manifestation of His power, whereas in Islam it is seen as the changing of God's nature. If God is perfect, and God's nature changes, how can He be perfect?
It would have to mean that He was not perfect before and is now perfect, which would not make Him God or that He was perfect before and is now not perfect, which would also not
make Him God.
If God is perfect, then there is no need for Him to become anything, so that He can grant us salvation. Since He's already perfect, loving and merciful, why would He need to change His nature to a weaker, human nature to grant us salvation? That's a contradiction. Therefore, when the Christian argues that God changing His nature makes Him perfect, this is not the case. They need to think before they speak.
If God is perfect and you add something to His nature, does that mean He was not perfect before? Which means He was not a God before this addition?
If God is perfect and you add something to His nature, that would mean He lacked something before and thus could not be a God.