The key words here are righteous and righteousness, which each occur twice and are each applied both to the Lord and to his testimonies. God’s testimonies mirror his righteousness, which is true both in fact and in the structure of the Tsadde section:

137 Righteous are You, O Lord
138 Your testimonies are righteous
142 Your righteous is everlasting
144 The righteousness of Your testimonies is everlasting

God’s testimonies are, as Alexander Maclaren says, “a true transcript of that Divine righteousness.” Even so, as God is eternal, so must his word be: “The beam must last as long as the sun.”
The righteousness of God and his testimonies calls up a righteous response in the psalmist as well, as we see in the section’s remaining verses: it inspires him to zeal (139), love (140), obedience (141), delight (143).

We can understand the word “pure” in vs. 140 as “refined” or “tested by fire” (ESV and NRSV, “well tried”; NIV, “thoroughly tested”). God’s testimonies “have stood sharp tests, and, like metal in the furnace, have not been dissolved but brightened by the heat. They have been tested, when the psalmist was afflicted and found them to hold true” (Maclaren). They are“without dross,” comments Andrew Bonar:

Pure gold. . .is said to be so fixed that an ounce of it set in the eye
of a glass-furnace, for two months, did not lose a single grain.

Joshua testified to Israel (Josh. 23:14), and they said Amen to the
testimony, tht not one good thing had failed of all that the Lord had
spoken. His word of promised and of threatening cannot deceive,
cannot disappoint, cannot come short of its declarations.

God’s testimonies makes it through the fire; no word is lost, no promise unfulfilled, regardless of the difficulties faced by his people. And because we can trust it, we love it. “The unadulterated word has a right to the undivided heart” (W. Graham Scroggie).

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