I'm certainly not an expert in Egyptian mythology, but I believe Akhenaten is the first known monotheist, which is why the later Egyptians rejected his reign so vociferously.

The separate elements of Christianity, such as the beliefs in a supreme God and in the soul's immortality are of course found in many religions. The question is whether the heart of the Christian narrative is a garbled version of earlier stories.

The solar cycle, as in the sun's apparent rise and fall and "rebirth" gives us the plot of all the dying and rising god myths, including Christianity's. It gives us the core of the passion narrative, if not, of course, its details such as the Judean setting or the method of execution.

It's like adapting the core of an American TV show to a foreign audience, with foreign actors and interests and a different setting.