The number of people is irrelevant
Appeal To Widespread Belief (Bandwagon Argument, Peer Pressure, Appeal to Common Practice):
However, if you know WHY "so many religious people testify about the good that religion brings" AND their reasons are valid, please do tell us
This is not quiet an appeal to widespread belief. Because I have an assumption that people realize what they are going through. Therefore religious people should know the effect on religion upon them, thus they are "experts" on the effects of religion. Ergo, this is not merely an appeal to widespread belief.