'Sign' in the Bible

If to others I am not an Apostle, at least I am one to you: for the fact that you are Christians is the sign that I am an Apostle.

For this reason it is right for the woman to have a sign of authority on her head, because of the angels.

For this reason tongues are for a sign, not to those who have faith, but to those who have not: but the prophet's word is for those who have faith, and not for the rest who have not.