From my friends at SerenityFountain.org
Question: Those who are tolerant in religious matters say, “When ‘Isa comes back, he will spread true Christianity. We will then work shoulder to shoulder with Christians to annihilate irreligiousness.” Will Hadrat ‘Isa spread Christianity?
It is unthinkable and very wrong. When ‘Isa alaihis-salâm comes back to earth, he will wipe out Christianity and will spread Islam as a member of this Ummah. The purport of two hadith-i sharifs is as follows:
(‘Isa will come as a follower of my religion.) [Imam-i Ahmad]
(‘Isa will descend from the sky as a just judge. He will break the cross [he will wipe out Christianity], kill pigs [he will forbid pork], and prohibit everything other than Islam.) [Bukhari, Muslim, Abu Dawud, Tirmidhi, Ibn Abi Shayba]