i don't see how we could possibly be the only life in the universe. i'd say it's pretty arrogant to even consider that out of all the planets in all the solar systems in all the galaxies that only the Earth cultivated life.
as for the effect of discovering alien life on religion, i'm pretty sure it won't do much to the 'true' believers. i think it would definitely have an effect on those that are casual believers or those that already question their religion.
i think it might also lead to a huge growth in scientology(if it still exists at that point)because of the whole 'alien' connection.