Jamieson, Fausset & Brown

Bring forth therefore fruits--the true reading clearly is "fruit"; meet for repentance--that is, such fruit as befits a true penitent. John now being gifted with a knowledge of the human heart, like a true minister of righteousness and lover of souls here directs them how to evidence and carry out their repentance, supposing it genuine; and in the following verses warns them of their danger in case it were not.