“Don’t cast the first stone” doesn’t mean what you think it means. This Biblical account has become the foundation for a pop-culture criticism to “not cast the first stone.” But what our society means by this phrase is, “because you have sin you can’t accuse others of wrong doing.” That is not what is being taught here.
The context: An adulterous woman is taken before Jesus by the religious leaders.(John 8:1-11) These leaders point out that according to the law she should be stoned but, they wanted to get Jesus perspective on the matter. The trap was set. They knew that a condoned stoning would be an unpopular choice however letting this woman get away with adultery would also be wrong. (The hypocrisy here of only bringing the woman is so ridiculous its unbearable.) Jesus wins the day with grace and mercy by saying, “Let whoever has no sin cast the first stone.” The religious leaders leave the woman standing before Jesus who tells her, “go and sin no more.”