By their fruits you shall know them...the prophets, that is

This verse is commonly misquoted as a reference to knowing Christians by their fruits. Mormons love it, as they think their fruits (good works) prove that Mormonism is true. But that's not what Matthew 7 is talking about. Matthew 7 is talking about prophets.

Jesus introduces His teaching this way:

15 "Watch out for false prophets, who come to you in sheep's clothing but inwardly are voracious wolves. 16 You will recognize them by their fruit."Matthew 7:15-16

Again, this verse is not about judging whether someone is Christian or not by their fruit, but judging prophets by their fruit.