Believe it or not, the thought that alien life might exist is actually a very old idea. In the 4th-3rd century B.C., the Greek philosopher Epicurus taught that “we must believe that in all worlds there are living creatures and plants and other things we see in this world.”
The debate began to rage when Aristotle rose against this teaching around the same time. Most early Christian scholars followed Aristotle’s lead. It wasn’t until over a thousand years later in the 13th century A.D. that a handful of Christian scholars wanted to talk about the topic. While all of these leaders rejected the notion of life on other planets, they at least wished to enter the conversation.
That wish finally came true in the year 1277 when Bishop Etienne Tempier condemned the Aristotelian belief that there could not be other worlds, because such a teaching could be construed to say that God wasn’t truly omnipotent. Because of this, theologians could now more openly converse about the possibility of extraterrestrials.
But as is often the case, the church moves slowly—as evidenced by the possibility that this conversation could still be perceived as controversial today. A serious conversation about the possibility of extraterrestrials has been going on for at least 2,000 years. If we ever actually discover life on another planet (even just a bacteria!) and we find that we don’t know what to do with that revelation from a theological standpoint, that’s more or less on us for not being willing to exercise some theological thought in that direction.
For detailed information on the progression of this debate throughout the early centuries, see: Crowe, Michael J. “Introduction: Before 1750.” The Extraterrestrial Life Debate: 1750-1900, Dover Publications, Inc., 1999.