This is a quite extraordinary quote from Dietrich Bonhoeffer about the Mosaic Law. He writes these lines in his exposition of Matthew 5:17-20.
The is the law of the Old Covenant, not a new law, but the one old law, to which the rich young man and the tempting scribe were referred as the revealed will of God. It becomes a new commandment only because Christ binds his disciples to the law. His concern is not for a “better law” than that of the Pharisees. It is one and the same, it is the law which must remain and be carried out in every letter until the end of the world, which must be fulfilled to the letter. His concern really is for a “better righteousness.” Those who do not have this better righteousness will not enter the kingdom of heaven. This will be because they have dispensed themselves from following Jesus, who referred them back to the law. But no one is able to achieve this better righteousness except those addressed here, those called by Christ. Christ’s call, Christ himself, is required for that better righteousness (116).
Can the preceding statement be appropriately summarized with the now famous dictum of E.P. Sanders [albeit revised for our purposes]: “What Jesus thought was wrong with Judaism was that it wasn’t Christianity”?