The Pharisees promoted obedience not only to the written law of the Old Testament, but also to the oral law. The oral law were the traditions that were developed over the centuries as Israel tried to apply the scriptures to the ever changing life in Palestine. At its best it was an attempt to help people keep God's law. At its worst it was a man-made religion of works that tried to earn God's favor. The Pharisees were greatly respected by most of the Jews. The Pharisees refused to associate with those that they regarded as sinful people. They accused Jesus of blurring the lines between the righteous and the unrighteous.
Jesus came to call sinful people - Mark 2:17
Jesus made it clear that the reason he came was not to enjoy the company of the righteous people; but instead he came to call sinful people to respond to the preaching of the good news of God.
Fasting - Mark 2:18
Fasting was required only once a year in Judaism, on the Day of Atonement (Lev.16:29-31). In Jesus' day, however, many Jews fasted much more frequently. Fasting, especially when combined with prayer, was a powerful way of expressing repentance or neediness. Some in the crowd appealed to Jesus because his disciples did not seem to be practicing fasting. As their leader Jesus was expected to guide them into sacred practices. In Matt. 6:16-18 Jesus seems to have assumed that his followers would fast.
The bridegroom - Mark 2:19-20
Jesus responded by comparing himself to a bridegroom. Jesus expected his disciples to be rejoicing while he was with them. When he was gone there would be time for fasting.
Making all things new - Mark 2:21-22
In two short parables Jesus stressed that he was making all things new. New, unshrunken cloth and new wine were pictures of the new day that was dawning in the history of God's people. While Jesus, the Messianic King, was with them there was no place for the rituals of the earlier days. In future days when the king was no longer in their midst and the kingdom was not yet present in its fullness there would be a time for fasting.
Picking a little grain on the Sabbath - Mark 2:23-24
The Pharisees challenged Jesus because his disciples picked some grain to eat on the Sabbath. The issue was not stealing. Deut.23:25 allowed this kind of picking a little grain from someone else's field as long as you didn't try to harvest the crop. Similarly, harvesting a crop was forbidden on the Sabbath by Exodus 34:21. The legalism of the Pharisees went further than the Old Testament law and claimed that this picking of some grain was the same as a harvest violation of the Sabbath.
The Son of Man is Lord even of the Sabbath - Mark 2:25
Jesus responded by referring to David and intensified the dispute beyond anything the Pharisees imagined in their original question. David was the anointed king and as such was privileged to give the temple bread to his followers (1 Sam. 21:1-6). Jesus was the anointed Messianic King. The disciples of Messiah were allowed to eat this grain. Notice that in the parallel passage in Matt. 12:6, 7 the thought is added that "one greater than the temple is here". Jesus was greater than the temple and as this paragraph in 2:28 concludes, The Son of Man is Lord even of the Sabbath. Jesus fulfilled all the shadows and types of the Old Testament. Jesus' healing on the Sabbath was a reaffirmation of the purpose of the Sabbath. It was intended for humanity's nurture and well-being not for its enslavement.
Abiathar - Mark 2:26
Mark mentioned Abiathar. It was actually Ahimelech, Abiathar's father, who was high priest at that time (1 Sam. 21:1). Abiathar became the far more famous of the two. It seems that Mark uses Abiathar's name as a way to identify the general period of history.