As a Christian, I really appreciate Keel, which is why I'm giving the book four stars. I don't think he was far from the kingdom with his Unified Theory of the Paranormal, and I believe it for the most part, with a few tweaks, it is an accurate account of the phenomena that occur in the world and usable.
However, I must say his argumentation for thinking that Jesus was just another myth like any other myth is now thoroughly dated. Very few, if any, in academia still argue this. Sure, Christianity has mythic elements, but that's not to discredit it. In fact, it is as Tolkien and Lewis contended, "Christianity is the true myth." Christianity did not occur in a vacuum, but in a world that shared mythic stories and had real eschatological expectations of a coming Messiah. The fact that Christianity has mythic elements makes it more plausible, not less. Especially with the historical evidence of the resurrection of Jesus.
Also, I have a slight disagreement with Keel that angels and demons are just one of many Ultraterrestrials who reside in the Superspectrum. He's close, but I would contend that behind all of UT who reside in the Superspectrum are what the Bible refers to as spiritual beings (elohim). There are various kinds of spiritual beings from angels, fallen angels, demons, elemental spirits, principalities, archangels, etc.
But, overall, thought-provoking read. I appreciate Keel's work.