Because there is not the smallest part of the law, not a single jot or tittle, that will be annulled, abolished, destroyed, or set aside, until the very end of time, then God’s commandments given in the Old Testament all remain in force for Christ’s disciples in this new age that begins with his coming into the world. Christ could not have set a higher value on them or more highly honoured the Old Testament.
Given such strong words directly from the Lord himself, how can any say that the Sermon on the Mount is not binding on all Christians in all ages? Early dispensationalists taught that the sermon was only applicable to Jews in the kingdom. According to dispensational theology, when Israel rejected the kingdom that Christ offered, the kingdom was put on hold and it will be established again only during a thousand-year millennium at the end of the gospel age. Since the Sermon on the Mount was given at the time when the kingdom was being offered, it belongs to it and defines an ethic only for those living in the kingdom. It does not therefore apply to Christians living before that time. (More recent dispensationalists have modified this view and apply the sermon more generally.) There is absolutely no qualification of any of this teaching by Christ, and he does not use the term ‘kingdom of heaven’ other than as a description of the church in all times. The way to enter the kingdom is not to be born into it, but to be poor in spirit, to seek first the kingdom and his righteousness, and to seek to do the will of the Father in heaven. These are the standards that will be applied not just to some but to all who have rejected them on the day of judgment (Matthew 7:22). Peter much later calls the church a holy nation using the same metaphor of a kingdom (1 Peter 2:9). These are the standards Christ requires of every follower in every age.