But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children's children; The mercy of the Lord is from everlasting to everlasting - חסד chesed signifies more particularly the exuberant goodness of God. This is an attribute of his nature, and must be from everlasting to everlasting; and hence, his righteousness (צדקת tsidketh) - his merciful mode of justifying the ungodly, is extended from one generation to another.
Unto children's children - It is still in force, and the doctrine of reconciliation through Christ shall continue to be preached till the conclusion of time.