The ancient Egyptians were
not Christians. Christianity emerged centuries after the decline of ancient Egyptian civilization.
Ancient Egyptians practiced a polytheistic religion with a complex pantheon of gods and goddesses, elaborate funerary rituals, and a focus on the afterlife. Christianity didn't arrive in Egypt until the Roman period, well after the era of the pharaohs and the great pyramids. While Egypt did become heavily Christianized in the centuries following the Roman conquest, this was a distinct period from the ancient Egyptian civilization.
So, to be clear: Ancient Egyptians practiced their own indigenous religion, entirely separate from Christianity.