malachi 3:14 Ye have said, It is vain to serve God: and what profit is it that we have kept his ordinance, and that we have walked mournfully before the LORD of hosts? - Free Bible Online

      14. what profit . . . that we . . . kept, &c.--(See on Mal 2:17). They here resume the same murmur against God. Job 21:14, 15; 22:17 describe a further stage of the same skeptical spirit, when the skeptic has actually ceased to keep God's service. Ps 73:1-14 describes the temptation to a like feeling in the saint when seeing the really godly suffer and the ungodly prosper in worldly goods now. The Jews here mistake utterly the nature of God's service, converting it into a mercenary bargain; they attended to outward observances, not from love to God, but in the hope of being well paid for in outward prosperity; when this was withheld, they charged God with being unjust, forgetting alike that God requires very different motives from theirs to accompany outward observances, and that God rewards even the true worshipper not so much in this life, as in the life to come.
      his ordinance--literally, what He requires to be kept, "His observances."
      walked mournfully--in mournful garb, sackcloth and ashes, the emblems of penitence; they forget Isa 58:3-8, where God, by showing what is true fasting, similarly rebukes those who then also said, Wherefore have we fasted and Thou seest not? &c. They mistook the outward show for real humiliation.

JFB.