Albert Barnes Commentary Matthew 12:4-5

"how he entered into the house of God, and ate the showbread, which it was not lawful for him to eat, neither for them that were with him, but only for the priests? Or have ye not read in the law, that on the sabbath day the priests in the temple profane the sabbath, and are guiltless?" — Matthew 12:4-5 (ASV)

House of God. This referred to the tabernacle, as the temple had not yet been built.

In the law. This refers to the law of Moses.

Profane the sabbath. He referred them to the conduct of the priests also. On the sabbath days, they were engaged, as well as on other days, in killing beasts for sacrifice (Numbers 28:9–10). Two lambs were killed on the sabbath, in addition to the daily sacrifice.

The priests had to be engaged in slaying them and making fires to burn them in sacrifice. However, kindling a fire was expressly forbidden to the Jews on the sabbath (Exodus 35:3). They were required to skin them, prepare them for sacrifice, and burn them. They did what, for other persons to do, would have been profaning the sabbath.

Yet they were blameless. They did what was necessary and commanded. This was done in the very temple as well, the place of holiness, where the law should be most strictly observed.