For centuries, Bible commentaries have tried to make sense of Genesis 6:1–7 and the identity of “the sons of God” and the “Nephilim.” Several theories have been put forth: (1) fallen angels who cohabitated with earthly women, (2) the godly descendants of Seth (“sons of God”) who intermarry with unbelievers (“daughters of men”), (3) dynastic rulers who are often described as “gods” (Heb.: elohim), and (4) extraterrestrial aliens. The “ancient astronaut view was first popularized by Erich von Däniken in his books Chariots of the Gods and Gods From Outer Space, but the theoretical stage for the theory was set in 1953 by Desmond Leslie’s (1891–1965) science fiction novel Flying Saucers Have Landed.
The theory goes like this. A number of modern-day UFOlogists believe the Bible is an ancient record of alien visitations misinterpreted as the work of God or gods. Modernist (liberal) theologians taught that the Bible must be “demythologized” of “the belief that the Biblical people were frequently visited by superior beings from another world,” what we and the biblical writers would call God and angels. While there is a kernel of truth in a biblical story, the modernist insists that most of the surrounding story is pure myth. The UFOlogist puts a new twist on the Bible by taking the biblical writers seriously without the need to demythologize the text. Barry H. Downing, a Presbyterian minister, writes the following in The Bible and Flying Saucers:
“It is not clear to me how we can demythologize the Biblical material in the name of science when in fact modern science seems quite convinced that in all probability there are in the universe many advanced civilizations involved in space travel. We cannot begin our Biblical studies with the assumption that the Biblical people were not visited by superior beings from another world; at least we cannot do so in the name of modern science."
Maybe the biblical writers were on to something, Downing theorizes. They just misinterpreted the phenomena, attributing “miracles” to God rather than to aliens. If these ancient peoples had our understanding of science, they would have immediately made the connection.
Downing has applied Altizer’s “death of God” thesis to his belief that the biblical writers are really describing UFO phenomena. Altizer was all the rage in the 1960s. The April 8, 1966 cover of Time magazine set the tone for what we are experiencing today by UFO religionists. “The moment has arrived,” Altizer wrote, “to engage in a radical quest for a new mode of religious understanding. The first requirement of such a quest is a forthright confession of the death of God in Christendom, a full acknowledgment that the era of Christian civilization has come to an end, with the result that all cognitive meaning and all moral values that were once historically associated with the Christian God have collapsed.” The death of God thesis was applied by Downing to the biblical text which made room for his theory that the biblical writers had not encountered God but extraterrestrial beings with superior science. “Miracles” were now easily explained as advanced technology unknown to ancient earthlings.
 “For proponents of the ancient astronaut and astrogenesis theories, the ‘sons of God’ or even the Nephilim refer to extraterrestrial visitors to the earth.”(Gary Bates, Alien Intrusion: UFOs and the Evolution Connection [Green Forest, AR: Master Books, 2004], 350).
 Erich Von Däniken, Gods from Outer Space: Return to the Stars or Evidence of the Impossible? (New York: G. P. Putnam’s Sons, 1968), 45.  Desmond Leslie (with George Adamski), Flying Saucers Have Landed (London: Werner Laurie , 1953).
 Barry H. Downing, The Bible and Flying Saucers: An Inquiry into Some Possibilities (Philadelphia, J. P. Lippincott, 1968), 8. When Downing wrote this book, he was the assistant pastor of Northminster Presbyterian Church in Endwell, New York. He received his B.D. degree from Princeton Theological Seminary. Also see http://archive.anomalies.net/Text-Archive/txt2/1287.ufo
 Downing, The Bible and Flying Saucers, 8.  Thomas J. J. Altizer, Mircea Eliade and the Dialectic of the Sacred (Philadelphia, PA: Westminster Press, 1963), 68.