Adam Clarke on Psalms 119:142

Thy righteousness is an everlasting righteousness - The word צדק tsedek is a word of very extensive meaning in the Bible. It signifies, not only God's inherent righteousness and perfection of nature, but also his method of treating others; his plan of redemption; his method of saving others. And the word δικαιοσυνη, which answers to it, in the Septuagint and in the New Testament, is used with the same latitude of meaning, and in the same sense; particularly in that remarkable passage, Rom 3:25-26 (note), where see the notes. Thy merciful method of dealing with sinners and justifying the ungodly will last as long as the earth lasts; and thy law that witnesses this, in all its pages, is the truth.