Many people have actually dabbled in several religions before ending up in the one they're currently in. Alternatively, they may have studied other religions to

Many people have actually dabbled in several religions before ending up in the one they're currently in. Alternatively, they may have studied other religions to some extent to reassure themselves of the "correctness" of their original religion. In that sense they are familiar with other beliefs and probably feel that they have rejected those other systems on their own merits. In that framework, the 519 of 520 argument does make since, exclusive of the monotheistic prohibition against other gods. I think what you are saying is that a Christian (for example) would say, "of course I don't believe in other god X, because I'm not allowed to". The 519 of 520 argument depends on whether the monotheist can also reject those other gods on their own merits.