It is very normal to believe in "prior" religions. It is in our books. Who ever says the opposite is trying to create problems between religions. Some true believes have been diverted gradually for some reason. But if we folow our book - the holly Quran - and Hadeeth, Christians have always been refered to as "Ansar" (supporters) and "the People of the 'holly' Book".

Unfortunately, some Churches did not recognize Mohammad PBUH as a prophet