I say that has already been done. You can look around and compare the results. Buddhists youth have gatherings where they sing songs and fellowship together. The Soka Association has a very nice building, strong small groups, great bonding. Muslims have gatherings for their youths with slogans like 'Love Islam". I do not know about Hindus. This is a free world and the methods of delivery are converging. As to whether they will convert back it all depends on why they like that religion in the first place. If they like the aircon, the excitement, the hard rock atmosphere, the relationships they can find, then they may convert if they find a more exciting place. That is probably the basis of your thoughts. If they like the idea that God, so distant and unknown in the other religions, is so near and close to you in Christianity, then there is no way they can convert because that sort of reality is not found in Buddhism, Hinduism and to a certain extent, not found in Islam too (God, yes but personal God, no). However all these are theoretical. What is important (at least should be important) is why are you focusing on this aspect and not on the more crucial aspect of living - is there God and is He found in Christianity? Because if you believe there is no God, then the rest is not important because it becomes doing good, living good and being good. That you can do without the need for religion.