Albert Barnes Commentary Psalms 89:2

"For I have said, Mercy shall be built up for ever; Thy faithfulness wilt thou establish in the very heavens." — Psalms 89:2 (ASV)

For I have said — The Septuagint and the Latin Vulgate render this, “You have said,” which is more in accordance with what the connection seems to demand, but the Hebrew will not admit of this construction. The true meaning seems to be that the psalmist had said—that is, he had said in his mind; he had firmly believed; he had so received it as a truth that it might be spoken of as firmly settled or as an indisputable reality. It was in his mind one of the things whose truthfulness did not admit of a doubt.

Mercy shall be built up for ever — The mercy referred to—the mercy manifested in the promise made to David. The idea is that the promise would be fully carried out or verified. It would not be like the foundation of a building which, after being laid, was abandoned; it would be as if the building for which the foundation was designed were carried up and completed. It would not be a forsaken, half-finished edifice, but an edifice fully erected.

Your faithfulness you shall establish — In the matter referred to—the promise made to David.

In the very heavens — Literally, “The heavens—you will establish your faithfulness in them.” That is, the heavens—the heavenly bodies—so regular, so fixed, so enduring, are looked upon as the emblem of stability. The psalmist brings them thus before his mind, and he says that God had, as it were, made His promise a part of the very heavens; He had given His faithfulness a place among the most secure, and fixed, and settled objects in nature. The sun in its regular rising; the stars in their certain course; the constellations, the same from age to age, were an emblem of the stability and security of the promises of God. .