You don't say whether you are playing with a flatpick, fingerpicks, or with just your fingers. It may be that you simply are not loud enough, especially since they have become accustomed to singing with an organ. I would suggest that you "amplyfy" your guitar if possible (just put a mic next to it if you have one available in the church). These folks may also be accumstomed to a choir "director." Increasing the volume may help them stay with you. I agree with everything else the others have said. Jim