Leibniz on extraterrestrials

If someone else came from the moon in some extraodinary machine, like
Gonsales, and told us credible tales about his homeland, we would take him to
be a lunarian; and yet we might grant him the rights of a native and a citizen
in our society, as well as the title *man*, despite the fact that he was a
stranger to our globe; but if he asked to be baptized, and to be regarded as a
convert to our faith, I believe that we would see great disputes arising among
the theologians. And if relations were opened up between ourselves and these
planetary men--whom M. Huygens says are not much different from men here--the
problem would warrant calling an Ecumenical Council to determine whether we
should undertake the propagation of our faith in regions beyond our globe. No
doubt some would maintain that the rational animals from those lands, not
being descended from Adam, do not partake of redemption by Jesus Christ; but
perhaps others would say that we do not know enough about all the places that
Adam was ever in, or about what became of his descendants--for there have been
theologians who have thought that Paradise was located on the moon. Perhaps
there would be a majority decision in favour of the safest course, which would
be to baptize these suspect humans conditionally on their being baptizable.
But I doubt if they would ever be found acceptable as priests of the Roman
Church, because until there was some revelation their consecration would
always be suspect, and that, according to the doctrine of that Church, would
expose people to the danger of unintentional idolatry. Fortunately, we are
spared these perplexities by the nature of things; but still these bizarre
fictions have their use in abstract studies, as aids to a better grasp of the
nature of our ideas.

Leibniz, New Essays on Human Understanding, 314