Charles H. Spurgeon on Psalms 119:142

Thy righteousness is an everlasting righteousness. Having in a previous verse ascribed righteousness to God, he now goes on to declare that that righteousness is unchanging and endures from age to age. This is the joy and glory of the saints, that what God is he always will be, and his mode of procedure towards the sons of men is immutable: having kept his promise, and dealt out justice among his people, he will do so world without end. Both the righteousnesses and the unrighteousnesses of men come to an end, but the righteousness of God is without end.

And thy law is the truth. As God is love, so his law is the truth, the very essence of truth, truth applied to ethics, truth in action, truth upon the judgment seat. We hear great disputes about, "What is truth?" The holy Scriptures are the only answer to that question. Note, that they are not only true, but the truth itself. We may not say of them that they contain the truth, but that they are the truth: "thy law is the truth." There is nothing false about the law or preceptory part of Scripture. Those who are obedient thereto shall find that they are walking in a way consistent with fact, while those who act contrary thereto are walking in a vain show.