Albert Barnes Commentary Psalms 103:18

"To such as keep his covenant, And to those that remember his precepts to do them." — Psalms 103:18 (ASV)

To such as keep his covenant - To such as adhere to the arrangements of his covenant, or who are faithful on their part. God will be faithful to his part of the covenant; and where there is fidelity on the part of his people, the blessings implied in the covenant will be conferred on them and on their children.

The promise is ample, and the fidelity of God is certain. However, it is still true that in those promises, and in that fidelity, it is implied that his people on their part must be faithful also, or the blessings will not be bestowed.

There are no promises of blessings to the unfaithful, nor do those who are unfaithful have any reason to hope that they or theirs will be partakers of the blessings of the covenant of mercy. Our only hope that we or our children will be partakers of the blessings of the covenant is found in the fact that we ourselves are faithful to God.

And to those that remember his commandments to do them - Who do not “forget” his law. If they do forget it, they have no right to expect the blessing. Obedience and fidelity are our only reasonable grounds of expectation of the blessing of God.