Packer on adoption and obedience
While it is certainly true that justification frees one forever from the need to keep the law, or try to, as the means of earning life, it is equally true that adoption lays on one the abiding obligation to keep the law, as the means of pleasing one’s newfound Father. Law-keeping is the family likeness of God’s children; Jesus fulfilled all righteousness, and God calls us to do likewise. Adoption puts law-keeping on a new footing: as children of God, we acknowledge the law’s authority as a rule for our lives, because we know that this is what our Father wants. If we sin, we confess our fault and ask our Father’s forgiveness on the basis of the family relationship, as Jesus taught us to do — “Father . . . forgive us our sins” (Lk 11:2, 4). The sins of God’s children do not destroy their justification or nullify their adoption, but they mar the children’s fellowship with their Father. “Be holy, for I am holy” is our Father’s word to us, and it is no part of justifying faith to lose sight of the fact that God, the King, wants his royal children to live lives worthy of their paternity and position.
— J. I. Packer, Knowing God, Downers Grove, IL: InterVarsity Press, 1993; p. 223