It is popular to say that Christians should live by moral standards, but that we have no right to call others to moral living. There is sliver of truth in this position, but also a thick slab of falsehood.
Some of the finer details of biblical morality are just for believers. Jesus gave the Sermon on the Mount to his disciples, not to the world at large (Mt 5:1-2). The prophets did not quote the Mosaic Law to foreign nations. The Law of Moses was for God’s chosen people, not for others.
Both Jesus and the prophets condemned sin in the nations. The prophet Amos is a good example of this. He condemned all of the nations that surrounded Israel. What he condemned them for was not for violations of the Mosaic Law, but for violations of more basic moral principles that any ought to be able to see. He quoted the Law only to Judah and Israel. But he condemned the sins of the nations as well. Certain moral principles are so basic, so much a part of nature, that no one has any excuse to ignore them.
A basic principle like the sanctity of human life is demanded of all people in all nations at all times. This principle is enshrined in the covenant with Noah as well as in all the other biblical covenants. It applies to all nations, not just to Israel. It applies to all people, not just Christians.
While Christians should not seek needless conflict with the world, we have no choice but to speak out on certain fundamental moral principles. As A. B. Bruce put it long ago, “If an ordinary prophet could not shirk the duty of censure, still less could the Christ. He must come with the fan of moral criticism in his hand, separating wheat from chaff.” (The Kingdom of God, p 188)
And what the Christ must do, his disciples must follow him in doing.
Some would ask, “Why can’t we always give positive messages? Why must we condemn sin?” As disciples of Jesus, as his spokesmen to the 21st century, we offer the good news of the gospel. But our version of the gospel must begin as his began, with a demand for repentance (Mt 4:17; Mk 1:14-15). If the demand for repentance is not prominent in our message, then we are not proclaiming his message; we are not really his disciples.