“The Son of God is one: our master Jesus Christ, coming forth in this world, died and was resurrected only once. Nor did he manifest himself elsewhere, nor has he died or been resurrected elsewhere. We should not imagine many worlds because we ought not imagine that Christ died and was risen often; nor should it be thought that in any other world without the knowledge of the Son of God that people would be restored to eternal life.”
Initia doctrinae physicae, Corpus Reformatorum 13 (Halle: Schwetschke, 1846; reprint, Frankfurt; Minerva, 1963) 1.221.
Cited by Thomas F. O’Meara, “Christian Theology and Extraterrestrial Intelligent Life” Theological Studies 60 (1999), pg. 6.