This is one of the best examples of the Egyptian origin of the cross as a symbol. It is both Ankh and Christian cross.

Anachrome "compatible" 3D image. (if you have red-cyan glasses, use them to see this in 3D stereo detail.)

Quote from Nativeborncal in en:Talk:Early Christian art and architecture (Not sure about its accuracy):

The ancient anhk symbol evolved into the key symbol of Christianity, because in Egypt it had a 2500 year pagan significance.
All over the rest of the Roman world, a cross was a simple tool of execution. Non-Egyptian Christians chose the fish symbol, or used an image of a youth Christ as a shepard, carrying a lamb over his shoulders. The Egyptian Christians smoothed the way to a general acceptance of the cross for Chistianity. I have posted a good example of that on the page with the bust image.