Can one be a true believer, a real Christian, and believe in UFO's. It depends. There are a number of prominent Christian patriots who believe that UFO's are actually piloted by DEMONS from another dimension, not space travelers.Some believe that the UFO occupants impregnate women, and that this is for the purpose of mixing species. They believe this is the same activity that went on, as shown in Genesis chapter 6. They say it's another sign that the end time is upon us, the Son's of God mixing with the sons of men.
I'm going to list a number of web sites here, and one particular video, Iron Mountain, which details that idea.