Jamieson, Fausset and Brown on Matthew 7:18

18. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit--Obvious as is the truth here expressed in different forms--that the heart determines and is the only proper interpreter of the actions of our life--no one who knows how the Church of Rome makes a merit of actions, quite apart from the motives that prompt them, and how the same tendency manifests itself from time to time even among Protestant Christians, can think it too obvious to be insisted on by the teachers of divine truth. Here follows a wholesome digression.