[According to some, the Sermon on the Mount] is the essence of Christianity, and Christ is the best of human teachers and examples. But he is not divine, for his function is only a human one, to teach and exemplify ethics. Christianity is essentially ethics.
What’s missing here? Simply the essence of Christianity, which is not the Sermon on the Mount. When Christianity was proclaimed throughout the world, the proclamation (kerygma) was not “Love your enemies?” but “Christ is risen!” This was not a new ideal but a new event, that God became man, died, and rose for our salvation. Christianity is first of all not ideal but real, an event, news, the gospel, the “good news.” The essence of Christianity is not Christianity; the essence of Christianity is Christ…
The Sermon on the Mount not only comes from Jesus but also leads us to Jesus. It does not divert us from Jesus to a set of abstract ideals, but its ideals lead us to Jesus. who alone can fulfill them in us, if we let him. The sermon is an arrow and Jesus is the bull’s eye, not vice versa.
Peter Kreeft, Back to virtue: traditional moral wisdom for modern moral confusion, Ignatius Press, 1992.