It’s so hard to understand or even to talk about grace and the law with any sense of clarity or soundness, but Bonhoeffer makes a point that helps me.
He says that, as Christ has come to fulfill and not destroy the law, Jesus’ call to us to be teachers and doers of the law is only the “greater righteousness” because Jesus stands between us and the law, for He was the one who fulfilled it in His life. Bonhoeffer says that “He is the righteousness of the disciples. By calling them he has admitted them to partnership with himself, and made them partakers of his righteousness in its fullness.” (emphasis mine) The thought which helps me is the idea that Jesus has made us partners in his righteousness, the better righteousness of the one who lived the law in its perfect interpretation.
The example which comes to mind is that of a law office. Often one sees signs about law offices called ‘the law offices of Johnson, Myers, and Kasinski;’ Johnson may be the founding and most senior lawyer while the others are just rookies. When one of the rookies is defending their case they have the power and the reputation of Johnson behind them, attached to them in partnership with them. I think it is the same with ourselves (if we are disciples) and Jesus. His life of obedience to the Father is connected to ours by his calling of us into his partnership, his fellowship. Though we must strive to be doers of the word in the way that Jesus was, we know that our striving is connected now to the striving of Jesus, which was perfect. We can rely on this connection in the same way that Myers or Kasinski can rely on their connection to the powerful and reputable Johnson.