Your mouth reveals it because it is written that:
“A good man brings good things out of the good stored up in his heart, and an evil man brings evil things out of the evil stored up in his heart. For the mouth speaks what the heart is full of.” – Lord Jesus Christ (Luke 6:45)
Please don’t be deceived by the “cover-up” quotes from the world such that “the heart is good though the mouth is bad” etc.
The Creator of both the heart & mouth revealed the truth regarding these as the verse above clearly states.