Catena Aurea on Matthew 3:7–10: Gregory the Great

Observe, he says not merely “fruits of repentance,” but “fruits meet for repentance.” For he who has never fallen into things unlawful, is of right allowed the use of all thing lawful; but if any hath fallen into sin, he ought so far to put away from him even things lawful, as far as he is conscious of having used unlawful things. It is left then to such man’s conscience to seek so much the greater gains of good works by repentance, the greater loss he has brought on himself by sin.

The Jews who gloried in their race, would not own themselves sinners because they were Abraham’s seed. “Say not among yourselves we are Abraham’s seed.”