I guess if you're looking for a formal answer, I'd say to the first they'd probably have to re-write the Bible. Believer assume that the Bible is wrapped around only human beings on earth and because it doesn't mention about other living beings on other planets, we (like years and years ago) believe we are in the center of the universe. The thought pattern hasn't changed much; we're just not throwing people in the pits and conquering nations for their lack of belief anymore (well, in America, at least..can't speak for other countries).
Believers would have to question whether God's promise is true for humans or does he have promises for
all his creation--aliens included. It probably make them wonder about Adam and Eve, the supposed start of the humankind. Also, it may make them wonder what category to put an alien in--is he an animal (then that would fit, God created animals. A plant?) If none of the above, where would aliens fit on this "human focused" faith.
For the second part, I honestly believe most atheist would be surprised. A lot of them will actually fall to their knees--not because they believe in an all powerful and all loving God--but because of fear. It is actually contradicts who God is when it comes to love when you have to bow down to him in fear or else. That, and not knowing what will happen--just like these days, is a reason to fear.
Some atheist might just laugh. I think these are the more solid of the bunch. They'd hold up their hands and just say "okay, take me away then."
I think, for me, I'd be fearful and abrephensive at first--given that's our first reaction to surprises. Then I would laugh too because it's just too funny that what can be ridiculous to one person is completely true to another--ironic.
I think in both cases, we feel the universe is wrapped us. If there was life elsewhere and we were not the center of the universe, we'd still have our morals regardless.
Bit of a weird question
But I was wondering, personally if it popped up on the news that we were contacted by aliens i'd be like "woah cool stuff!" but how would religious people react to this sort of news? would they think the same or riot over it? would this debunk something and make them angry?
I think it's only fair now to say, what if it was somehow proven that god does exist for all the non-religious people? proven in a way that you simply can't reject it.. i'm not religious but I guess my reaction would be "oh.. alrite"