For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies:

People's New Testament

15:18-20 Come forth from the heart. The emotional nature; the mind. Evil deeds are begotten of evil thoughts; evil words are the expression of these evil thoughts. These indicate a sinful heart and make a man sinful, or defiled.

Wesley's Notes

15:19 First evil thoughts - then murders - and the rest. Railings - The Greek word includes all reviling, backbiting, and evil speaking.

Jamieson-Fausset-Brown Bible Commentary

19. For out of the heart proceed evil thoughts-"evil reasonings"; referring here more immediately to those corrupt reasonings which had stealthily introduced and gradually reared up that hideous fabric of tradition which at length practically nullified the unchangeable principles of the moral law. But the statement is far broader than this; namely that the first shape which the evil that is in the heart takes, when it begins actively to stir, is that of "considerations" or "reasonings" on certain suggested actions.

murders, adulteries, fornications, thefts, false witness, blasphemies-detractions, whether directed against God or man; here the reference seems to be to the latter. Mark (Mr 7:22) adds, "covetousnesses"-or desires after more; "wickednesses"-here meaning, perhaps, malignities of various forms; "deceit, lasciviousness"-meaning, excess or enormity of any kind, though by later writers restricted to lewdness; "an evil eye"-meaning, all looks or glances of envy, jealousy, or ill will towards a neighbor; "pride, foolishness"-in the Old Testament sense of "folly"; that is, criminal senselessness, the folly of the heart. How appalling is this black catalogue!

Matthew Henry's Concise Commentary

15:10-20 Christ shows that the defilement they ought to fear, was not from what entered their mouths as food, but from what came out of their mouths, which showed the wickedness of their hearts. Nothing will last in the soul but the regenerating graces of the Holy Spirit; and nothing should be admitted into the church but what is from above; therefore, whoever is offended by a plain, seasonable declaration of the truth, we should not be troubled at it. The disciples ask to be better taught as to this matter. Where a weak head doubts concerning any word of Christ, an upright heart and a willing mind seek for instruction. It is the heart that is desperately wicked, Jer 17:9, for there is no sin in word or deed, which was not first in the heart. They all come out of the man, and are fruits of that wickedness which is in the heart, and is wrought there. When Christ teaches, he will show men the deceitfulness and wickedness of their own hearts; he will teach them to humble themselves, and to seek to be cleansed in the Fountain opened for sin and uncleanness.