If one religion can exist alongside the discovery of alien life, it doesn't mean all of them can. Beliefs differ in ideologies, some supporting the discovery of alien life, while others do not. Christianity is the largest single religion in the world, with 2.4 billion adherents, making up for 29% of religious populations.
It is also strict in its belief system that humans are made from God's image, and that there is only one God. Christianity is not supportive of alien life in its belief system. If the discovery of alien life is confirmed, it would mean the end of the largest and most practiced religion in the world.