True. That's why I said I don't read too much into it. Still, one way to confirm a causal relationship from otherwise superficial similarity, is to see if there's a plausible timeline for the symbol to have evolved.

For instance, the original symbols of Christianity were the fish and the "chi rho" (which looks like a P over an X - and if you squint a bit, like a man's figure with his limbs stretched on the aforementioned X-shaped Roman crucifix). However, early Christianity was further molded in the care of the Coptic church, which used the Ankh, known as the "crux ansata", as a mainline symbol. And as the Coptic church was instrumental in the development of Christianity, eventually the symbol further developed into the Crucifix we know today.

The interesting coincidence here - and the reason I say symbols outlive scripture - is that the Ankh wasn't just a random scribble, but one of the most important symbols in Egyptian religion - signifying both divine domain and... eternal life. As in, the eternal life offered by the god Osiris, himself risen from the dead and having the power of judgment over all souls... Sounds familiar, doesn't it? A bit too much to be a coincidence on it's own, I'd wager.