There's a chance that by the time we'll get invaded, we'll possess similar technology to that of extra terrestial life.
If we were to be invaded right now, it would be a one sided battle for aliens.
Oh yeah, here's a good read for people who think that aliens don't exist:
http://en.wikipedia.org/wiki/Fermi_paradox
If comes to religion related stuff....well, I'm thinking of two scenarios. Either the Church will state that these extra terrestial beings were created by hand of God, effetively changing whatever is in the Bible to Church's liking, which will take care of whatever "credibility" they have left..or that they simply won't accept that there is race of intelligent beings other than humans (or animals on Earth if we count these) and they will begin war themselves, even if aliens would come in peace.
...oh well, maybe religions won't exist anymore when we'll get invaded so it won't be a problem <- wishful thinking
