So, fun fact, St Augustine, who is considered one of the Church Fathers, explicitly argued that if the 'Antipodes' (i.e., southern continents not connected to Europe, Asia or Africa) actually existed and had humans living there, that would prove the Gospel was untrue.

The reason for this is as follows: Christians of his era believed that the reason God had allowed the Romans to destroy the Second Temple and push the Jews into exile was to prepare the men of all nations (as understood at the time) for the coming of the Gospel. The idea was that the Jews had taken the Old Testament, and the prophecies of the Messiah therein, across the whole world. Augustine argues that if the Antipodes contained human beings who had never had any kind of contact with Jews, and therefore no contact with the OT, and no contact with Christians, and therefore no contact with the New Testament, either, that must mean the Gospels are false. Why? Because there's no conceivable reason that a just God would have deprived entire civilisations of the chance of redemption.

Of course, we now know that at the time Augustine was writing (4th-5th century AD), there were literally millions of people who had never had the slightest contact with the Jews or Christians and, furthermore, wouldn't do so for another millennium. So, per Augustine's argument, all those millions were condemned to Hell (the concept of Purgatory didn't exist at this point, but condemning them all to no chance of Heaven, just because they were unfortunate to be born a long way away from Jersualem, is clearly also unjust). Either God is incredibly unjust and unmerciful, which means the Gospels are untrue, OR the Good News wasn't actually spread to all men, which must also mean that they're not true.

The upshot of this is that one of the Church Fathers has, in retrospect, irrefutably argued that the Gospels are untrue. The amount of special pleading required to make out that, actually, the Maori or the Easter Islanders or [insert any other uncontacted peoples here] had an opportunity to accept Christ and somehow missed it entirely is far beyond any sane interpretation of the evidence.

Now, as you might have noticed, this hasn't stopped people from believing in the Gospels. I don't see why the discovery of life on another world would dislodge people from a belief that is transparently false when nothing else has.