Due to their zeal to obey the law of God, the ancient Pharisees were well respected as pious men by the common people of Jesus' day. Yet, as many of their encounters with our Lord demonstrate, their fervency for the Mosaic law often led them to make the law into an onerous burden. In order to ensure that the prohibition of labor on the Sabbath would not be violated (Ex. 20:8–11), they developed an extensive list of what qualified as work that was prohibited on the Sabbath. This quickly became a list of minutiae that was nothing less than nonsensical. Things such as tying and untying knots or writing more than one letter of the alphabet were defined as labor that would break the Sabbath commandment. Of course, such things that would qualify as work under any commonsense definition of labor were also on this list.
The Pharisees erred in seeing the commandments as ends in themselves. Little time was spent focusing on the purpose of the law of God in revealing the Lord's character or the reasons behind the stipulations of the old covenant regulations, and more time was spent on such an exacting parsing of the lesser commandments that the weightier matters of the law were overlooked (see Matt. 23:23–24). With respect to the Sabbath in particular, the Pharisees, in their attempt to figure out each and every thing that might possibly be considered work in every circumstance, actually missed the purpose of the day of rest in providing for the overall well-being of men and women. The Pharisees had expanded the law against harvesting crops on the Sabbath far beyond its original intent to include any and all gathering of food from the fields (see Ex. 31:12–17; 34:21), so they were upset at the disciples of Jesus for casually picking isolated heads of grain to satisfy hunger and maintain vitality.
As noted, the Pharisees became so preoccupied with their rules for applying the law of Moses that they missed its purpose. That is what Jesus meant when He said, "The Sabbath was meant for man, not man for the Sabbath" (Mark 2:27). John Calvin mentions that God intended the Sabbath to be "advantageous" for people, not to injure them. A list of extrabiblical rules that kept hungry people from expending even the tiniest bit of effort on the Sabbath to feed themselves was never what the Lord intended. He gave His law to bless us, not to give us a series of regulations so complex and unwieldy that even regenerate people cannot make heads or tails of it.
We should look at the law and realize we cannot obey it perfectly for our justification. But the reason for our failure is not the impossibility of knowing the law itself but rather the presence of sin. Regenerate people are supposed to love the law of God (Ps. 119:97) and to find it a delight in their sanctification as they are conformed to the image of Christ. But if we treat God's law as an end in itself and not as the means to know God, we will not know the law rightly or delight in it as we should.