O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh. O generation of vipers - These are apparently severe words; but they were extremely proper in reference to that execrable people to whom they were addressed: the whole verse is an inference from what was spoken before.
Out of the abundance (περισσευματος, the overflowings) of the heart - Wicked words and sinful actions may be considered as the overflowings of a heart that is more than full of the spirit of wickedness; and holy words and righteous deeds may be considered as the overflowings of a heart that is filled with the Holy Spirit, and running over with love to God and man.