15 “Righteousness and judgment are the preparation of Your seat”. Your righteousness and judgment will appear in the end: they are now hidden. Of Your righteousness it is treated in another Psalm, “on the hidden things of the Son.” There will then be a manifestation of Your righteousness and judgment: some will be set on the right, others on the left hand: and the unbelieving will tremble, when they see what now they mock at, and believe not: the righteous will rejoice, when they shall see what they now see not, yet believe. “Righteousness and judgment are the preparation of Your seat:” especially in the Day of Judgment. What then now? “mercy and truth go before Your face.” I should fear the preparation of Your seat, Your justice, and Your coming judgment, did not mercy and truth go before You: why should I at the end fear Your righteousness, when with Your mercy going before You Thou blottest out my sins, and by showing forth Your truth fulfillest Your promises? “Mercy and truth go before Your face.” For “all the paths of the Lord are mercy and truth.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)