12:28 Which is the first commandment? - The principal, and most necessary to be observed. Mt 22:34; Lu 10:25.

Mark 12:28 Parallel Commentaries

The First and Great Commandment
It is "the first commandment," again, not only for antiquity, but for dignity. This command, which deals with God the Almighty must ever take precedence of every other. Other commandments deal with man and man, but this with man and his Creator. Other commands of a ceremonial kind, when disobeyed, may involve but slight consequences upon the person who may happen to offend, but this disobeyed provokes the wrath of God, and brings his ire at once upon the sinner's head. He that stealeth committeth …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 3: 1857

Controversy
The priests and rulers had listened in silence to Christ's pointed rebukes. They could not refute His charges. But they were only the more determined to entrap Him, and with this object they sent to Him spies, "which should feign themselves just men, that they might take hold of His words, that so they might deliver Him unto the power and authority of the governor." They did not send the old Pharisees whom Jesus had often met, but young men, who were ardent and zealous, and whom, they thought, Christ …
Ellen Gould White—The Desire of Ages

Woes on the Pharisees
It was the last day of Christ's teaching in the temple. Of the vast throngs that were gathered at Jerusalem, the attention of all had been attracted to Him; the people had crowded the temple courts, watching the contest that had been in progress, and they eagerly caught every word that fell from His lips. Never before had such a scene been witnessed. There stood the young Galilean, bearing no earthly honor or royal badge. Surrounding Him were priests in their rich apparel, rulers with robes and badges …
Ellen Gould White—The Desire of Ages