How were the previous prophets another religion?

Today I was in a discussion about previous prophets before Mohammed. Then we asked ourselves: Judaism and Christianity came before Islam so the prophets before Islam started were of a different religion unless the Jews and Christians used the prophets 'stories' to change them and make a religion. Someone also said that the previous prophets were muslims but the people changed the religion to suit their lifestyle. So my question are: Before islam started, Were the previous prophets originally muslims? if so then why did the people change them to christians and jews? Sorry if this all seems confusing. I am confused myself!