All of the New Testament authors were known from the beginning except Hebrews as far as we know. I have no idea where you got the 400 ad date. Papias confirmed the authors of the Gospels and he lived in the first and second century. Further, I know of nobody who claims an alien encounter that also claims there were scores of witnesses to the encounter,
You are still talking about fewer witnesses when compared to the UFO events. That they are not believed seems odd to me. People can believe a very small number of witnesses from 2000 years ago when those ancient texts have been in the hands of a clearly confused Catholic Church, but they can’t believe hundreds of men with solid credentials in our own society.