Psalm 103:17

But the mercy of the Lord is from everlasting to everlasting
upon them that fear him
In opposition to the frailty of man, the stability and duration of the mercy of God is observed. This reaches from one eternity to another; it is from everlasting in the heart of God: it appeared in the choice of the vessels of mercy; in the covenant of grace, which is founded upon it, and filled with it; every blessing of which flows from it, and therefore are called the "sure mercies of David". And it appears in time in the regeneration of God's elect, which is according to his abundant mercy; in the forgiveness of their sins, which is according to the multitude of his tender mercies; and in their whole salvation, which is by that, and not by works of righteousness; and will endure for ever, for the mercy of our Lord Jesus Christ is unto eternal life. It reaches from the world past to the world to come, as the Targum; and it is so "upon them that fear" the Lord; not with a servile, but a filial fear; a fear of the Lord and his goodness; which only is consistent with the grace and mercy of God, and a sense of it: not that the fear of God is the cause of mercy or grace; but, on the contrary, grace and mercy are the cause of the fear of God; which is a blessing of the covenant of grace, and one of the first things which appear in conversion; but this properly describes the persons who openly and manifestly share in the grace or mercy of God, and to whom he manifests it yet more and more; nor have any reason to believe they are the objects of it, until the true fear of God is wrought in their hearts; and, besides, this character may be given to show that the mercy and grace of God are not limited to the Israelites only, but belong to such of all nations that fear the Lord.

And his righteousness unto children's children;
not the essential righteousness of God, but rather his faithfulness in the performance of his promises, which he will not suffer to fail: the justifying righteousness of Christ is here meant; which is an everlasting one, and is unto and upon all them that believe, in all successive generations; which is meant by the phrase of "children's children", even the spiritual seed of Christ, the seed of the church, the seed of Israel; to all and each of which, in every age, the word of God comes, and his promises are fulfilled; and who are justified by and glory in Christ, their righteousness; and who are further described in the next verse, which shows that not the carnal seed of them that fear the Lord are meant.