Albert Barnes Commentary Malachi 3:14

"Ye have said, It is vain to serve God; and what profit is it that we have kept his charge, and that we have walked mournfully before Jehovah of hosts?" — Malachi 3:14 (ASV)

You have said, It is vain to serve the God - “as receiving no gain or reward for their service. This is the judgment of the world, by which worldly people think pious, just, sincere, strict men, vain—that is, especially when they see them impoverished, despised, oppressed, and afflicted—because they do not know the true goods of virtue and eternal glory, but measure all things by sight, sense, and taste.

“Truly, if the righteous did not have hope of another and better life, in vain would they afflict themselves and bear the afflictions of others. For, as the Apostle says (1 Corinthians 15:19). ‘If in this life only we have hope in Christ, we are of all men most miserable.’

“But now, hoping for another blessed and eternal life in return for the slight tribulations of this present life, we are the happiest of all men.”

And we have walked mournfully - Again they take up the words of Psalmists, meaning that they adopted the attire of mourners, going about mourning before God for their country’s afflictions.