When David and his men were permitted to eat the showbread, it was because they all, by virtue of having been on special assignment, had consecrated themselves and abstained from relations with their wives for the duration. In other words, they were ritually clean, and therefore—like the priests of the Temple—permitted to eat of the holy bread. The disciples of Jesus, like the companions of David, are on special assignment from the King and are living holy lives. More than even the companions of David, the disciples are attendants to the High Priest, the very dwelling of God in Flesh: they are priests serving the Temple of Christ’s Body (John 2:21), which is far greater than the Temple built in Jerusalem. Had the Pharisees noted this or believed in Jesus, they would have said nothing.
“I desire mercy, not sacrifice.” To “sacrifice” is to offer something to God, to make it holy; Jesus is reminding the Pharisees that God also desires them to recognize what is already holy, to live in the constant awareness of God’s supreme and imminent holiness. Were they doing so, they would not only see nothing wrong in the actions of the disciples but, with their hearts uncluttered by pride, they would see God Himself in Christ and know the beatitude of those who show mercy and receive it (Matthew 5:7).