Matthew 12:1-8 adds yet another example of Sabbath encounters Jesus had with the Pharisees. According to the Pharisees, the disciples reaped, threshed, and winnowed the grain; they were guilty of preparing a meal. What was the disciples' motivation? They were traveling, hungry, and had no place to prepare a meal. They were young and strong and could have fasted without harm, but because it was a Sabbath, Jesus drew attention to one of the Sabbath's main purposes. It is a day of mercy.
Christ draws His justification from I Samuel 21:1-6. He reasons that, if David under unusual circumstances could allay his hunger by eating bread consecrated for holy use, then the disciples could also legitimately provide for their needs in unusual circumstances. The emphasis here is on "unusual." How many times did David flee for his life and find himself hungry near the Tabernacle? It happened at least once, but even for a man of war like David, such situations occurred only rarely.
The overall lesson is that God does not intend His law to deprive but to ensure life. If the need arises, we should not feel conscience-stricken to use the Sabbath in a way that would not normally be lawful. Christ admits David's actions were not normally lawful, and neither were the disciples'—except for the circumstances. In this case, they were blameless BECAUSE A LARGER OBLIGATION OVERRULED THE LETTER OF THE LAW. In this circumstance, mercy is more important than sacrificing a meal. Holy bread or holy time can be used exceptionally to sustain life and serve God.
Christ takes advantage of the situation to teach another connected lesson. He draws attention to the extent of the priests' Sabbath labors in the Temple. Their work actually doubled on the Sabbath because of the number of sacrifices God required, yet they were guiltless. Why? They were involved in God's creative, redemptive work, as Christ explains in John 5, 7, and 9. They fulfilled a purpose of the Sabbath that someone had to do.
Because of the disciple's involvement in the work of God, circumstances dictated a profaning of the Sabbath. From this, we can understand that LOVING SERVICE IS GREATER THAN RITUAL FULFILLMENT. What is mercy? It is a helpful act where and when it is needed. It is an act of loving encouragement, comfort, pity, and sympathy for the distressed. It is the relieving of a burden.