I've never met a serious religious person who considered it an important part of their faith that there was no other intelligent life in the cosmos. Some had given it some thought, and arrived at the same conclusions as Lewis. Most didn't really think it was important one way or another.
When you stop and think about it, if a deity had created only the Earth and its life, and not the entire universe or even the entire solar system, that wouldn't make all that much difference in our relationship to that deity, would it?
The most significant thing about discovering life forms outside the Earth is that it gives us another data point for analyzing what might be universal about life and biology and what might not be. Right now, we only have one data point, and that's not helpful.
If we communicated with other civilizations, it might give us similar insights on religion and cultures. A religion in common among many separate civilizations would be something to give one pause, as would universal atheism. The most likely result, I fear, is that we would find as many widely-ranging belief systems as there are civilizations.