There was nothing narrow in such teaching; for although in Jesus' teaching the intimate relation of sonship toward God was offered only to those who should be of the household of faith, yet the door of the household of faith was open wide to all who would be willing to come in. Indeed Jesus himself died on the cross with the purpose of opening that door. Our Saviour did far more than teach men that they were already children of God; he came to make them children of God by his saving work.
In the second place, the Sermon on the Mount tells what kind of life is led by those who should have entered into the Kingdom and been made the children of God. That life is far more than obedience to a set of external rules; the purity which Jesus demanded is a purity of the heart. The life in the Kingdom is also far removed from all pretense; the children of God engage in prayer and good works not to be seen by men but to be seen by God. Finally, the life in the Kingdom is a life of perfect trust; all anxious thought for the morrow is banished, since God will care for his children.
One difficulty arises in the reading of the Sermon on the Mount. How can such an ideal be attained? It might be possible to obey a set of rules, like the rules of the Pharisees, but how is it possible for sinful men to attain purity of heart? The righteousness of the Kingdom of heaven exceeds by far the "righteousness of the scribes and Pharisees." How can such righteousness be attained?
The answer to this question was partly understood even by the first hearers of the Sermon on the Mount. The disciples of Jesus knew even then that Jesus alone could give them entrance into the Kingdom; they trusted in him already not merely as teacher but also as Saviour. But the answer to the question is far plainer to us; for we know the cross. The atoning death of Christ it was that gave men the kind of righteousness required for entrance into the Kingdom of God, for it gave them the righteousness of Christ himself. The significance of the cross was spoken of by our Lord even during his earthly ministry, but the full explanation of it was left to the apostles. The saving work of Jesus could be fully explained only after it had been done.