In several of those religious paintings the clouds are just that - clouds. Find them online and see for yourselves. They're not UFO-like or discs.

Also the likeliest explanation seen for the UFO-like things in the other paintaings is that the painters drew what they read in the Bible and in the religious experiences of the church fathers. That however does not explain what were those things.

It does however show that UFOs might be spiritual or material entities directly involved in our religions.