Jesus here cites Hosea 6:6, which argues that mercy is more powerful, and better than to sacrifice on the Sabbath. Thus it is lawful to violate the Sabbath rest. It was said that the world is supported by three things, 1) by the law, 2) by Divine worship, 3) by mercy.[1][2]
Chrysostom: " And because what He had said seemed hard to those that heard it, He again exhorts to mercy, introducing His discourse with emphasis, saying, But had ye known what that meaneth, I will have mercy and not sacrifice, ye would never have condemned the innocent."[3]
Jerome: " What I will have mercy, and not sacrifice, signifies, we have explained above. The words, Ye would never have condemned the innocent, are to be referred to the Apostles, and the meaning is, If ye allow the mercy of Achimelech, in that he refreshed David when in danger of famishing, why do ye condemn My disciples?"[3]
^John MacEvilly, An Exposition of the Gospel of St. John consisting of an analysis of each chapter and of a Commentary critical, exegetical, doctrinal and moral, Dublin Gill & Son 1879.
^Cornelius Cornelii a Lapide; Thomas Wimberly Mossman The great commentary of Cornelius à Lapide, London: J. Hodges, 1889-1896.