Matthew 12:7 - Ellicott's Commentary On The Whole Bible

I will have mercy, and not sacrifice. — Yet a third argument follows from the Old Testament (Hosea 6:6). The teachers or interpreters of the Law had failed to catch the meaning of the simplest utterances of the prophets. “Mercy and not sacrifice,” moral and not positive duties, these made up the true life of religion, and were alone acceptable to God. It was because they had inverted the right relation of the two that they had, in this instance, condemned those whom our Lord now declares to have been in this respect absolutely guiltless.