We should never cease speaking moral truth prophetically into the culture(s) in which we live ... proclaiming God's standard of right and wrong to which all people's conscience already bears witness. But we must remember that even if you were to persuade a few people of the truth of your moral cause you have still done nothing to advance the kingdom of God unless you proclaim the gospel with even greater clarity. Apart from grace it is the natural state of man to love darkness and hate the light (John 3:19, 20) so having them exchange a specific sin for some kind of moralism has done nothing beneficial, in any redemptive sense, for their soul. You have merely exchanged one sin for another. No amount of moral agreement with you or behavior modification will save their soul.
So while we must declare God's moral law we must take even greater efforts to declare the gospel of Jesus Christ, the only person to have fulfilled God's law - this must be our main focus. Only as the Holy Spirit germinates the seed of the gospel does true life begin. It is through this, and this alone that God's kingdom advances, because the word of the gospel is where God has promised to changes hearts ... and indeed moral advance will follow (1John 5:1-4). If you want to change the culture then remember, first and foremost, that people need a new heart. outward laws may restrain certain evils, but the inward writing of the law on the heart transforms the person. While politics certainly has its place, politics is small and limited in scope. On the other hand, Christ touches the whole man - the nature of the universe, the meaning of life and death and everything.