For I have said, Mercy shall be built up for ever. He assigns the reason why he perseveres in singing the Divine praises in the midst of adversities; which is, that he does not despair of the manifestation of God's loving-kindness towards his people, although at present they were under severe chastisement. Never will a man freely open his mouth to praise God, unless he is fully persuaded that God, even when he is angry with his people, never lays aside his fatherly affection towards them. The words I have said, imply that the truth which the inspired writer propounds was deeply fixed in his heart. Whatever, as if he had said, has hitherto happened, it has never had the effect of effacing from my heart the undoubted hope of experiencing the Divine favor as to the future, and I will always continue steadfastly to cherish the same feeling. It is to be observed, that it was not without a painful and arduous conflict that he succeeded in embracing by faith the goodness of God, which at that time had entirely vanished out of sight; -- this we say is to be particularly noticed, in order that when God at any time withdraws from us all the tokens of his love, we may nevertheless learn to erect in our hearts that everlasting building of mercy, which is here spoken of, -- a metaphor, by which is meant that the Divine mercy shall be extended, or shall continue till it reach its end or consummation. In the second clause of the verse something must be supplied. The sense, in short, is, that the Divine promise is no less stable than the settled course of the heavens, which is eternal and exempt from all change. By the word heavens I understand not only the visible skies, but the heavens which are above the whole frame of the world; for the truth of God, in the heavenly glory of his kingdom, is placed above all the elements of the world.

For I have said - The Septuagint and the Latin Vulgate render this, "Thou hast 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.
Thy faithfulness shalt thou establish - In the matter referred to - the promise made to David.
In the very heavens - literally, "The heavens - thou wilt establish thy 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 to 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. Compare Jeremiah 33:20-21.

Mercy shall be built up for ever - God's goodness is the foundation on which his mercy rests; and from that source, and on that foundation, acts of mercy shall flow and be built up for ever and ever.
Thy faithfulness shalt thou establish - What thou hast promised to do to the children of men on earth, thou dost register in heaven, and thy promise shall never fail.

For I have (b) said, Mercy shall be built up for ever: thy faithfulness shalt thou (c) establish in the very heavens.
(b) As he who surely believed in heart.
(c) As your invisible heaven is not subject to any alteration and change: so shall the truth of your promise be unchangeable.

For I have said,.... That is, in his heart he had said, he had thought of it, was assured of it, strongly concluded it, from the Spirit and word of God; he believed it, and therefore he spoke it; having it from the Lord, it was all one as if he had spoke it:

I have said--expressed, as well as felt, my convictions (2-Corinthians 4:13).

Establish - As firmly and durably as the heavens themselves.