New Defender's Study Bible Notes
12:3 Have ye not read. The Lord Jesus frequently referred to the Old Testament Scriptures in support of His teachings and actions (here it is from I Samuel 21:1-6). He also assumed that His listeners would be familiar with the Scriptures cited.
12:5 read in the law. Christ was not referring to a specific Scripture here, but to the fact that priests had to officiate at sacrifices on the Sabbath (Numbers 28:9) and also perform circumcisions on the Sabbath (John 7:22).
12:8 sabbath day. The Son of man, in fact, established the Sabbath day, when He “rested from all His work God created and made” (Genesis 2:3). As the Creator of all things (John 1:1-3), Christ surely had all authority over the Sabbath day, especially “to do well on the sabbath” (Matthew 12:12).