Albert Barnes Commentary Psalms 119:142

"Thy righteousness is an everlasting righteousness, And thy law is truth." — Psalms 119:142 (ASV)

Your righteousness is an everlasting righteousness - It never changes. The principles of your law, of your government, and of your method of saving people, are the same under all dispensations, in every land, in all worlds; and they will remain the same forever. Human governments change. Old dynasties pass away. New laws are enacted under new administrations. Customs change. Opinions change. People change. The world changes. But as God himself never changes, so it is with his law. That law is founded on eternal truth, and can never change.

And your law is the truth - It is founded on “truth;” on the reality of things. It is so essentially founded on truth, it springs so certainly out of truth, or out of the reality of things, that it may be said to be the truth itself. He who understands the law of God understands what truth is, for it is the expression and the exponent of that which is true.