The question is not whether we all desire to live in and to promote a moral society. The question is if the main business of the Church is to preach morality? To answer this question, may I draw your attention to something Martyn Lloyd-Jones preached on Ephesians chapter five. After having settled it that ethical teachings alone will not alter the morality of a nation, he affirms what is the God-ordained purpose of the church.
I would also state it as my firm conviction that such activities are not a part of the Church's business as such. And here I feel the confusion comes in as between the realm of the Church and the realm of the State. The Church is appointed and ordained of God; so is the State, says the Apostle in Romans 13 and elsewhere. They are both appointed by God. . . . The two realms are meant to work in their appointed ways, and there should never be any confusion between them. . . .
The Christian Church is not a moral agency. What is she then? She is a regenerating agency! She is not a moral agency; there are many other moral agencies; the Church is not, she is supernatural, she is divine, she is filled with the Spirit; she converts men, she regenerates men; I say she does it, she is used of God to do it. That is her realm.
He goes on . . .
The Church does not exist to produce good men. The Church exists to produce new men! And a new man is infinitely greater than a good man; you can be a good man without being a Christian at all. I repeat the word; the Church exists not to produce moral men, but to produce those who, in Scripture, are called saints. IF YOU DO NOT SEE THAT DISTINCTION, YOU DO NOT KNOW WHAT CHRISTIANITY IS! [emphasis mine] Certainly a Christian is a good man and a moral man, but if you simply describe him in those terms you are insulting him; you are not describing him. The Christian is a saint! He is a regenerate man! He is a child of God! He belongs to Christ, he is 'in Christ'! We are not in the realm of morality and goodness, we are in the realm of sainthood and of the Spirit! And therefore I argue that the acceptance of a moral-agency role by the Church means that the Church is misrepresenting her own message.This is a critical issue for us today. If we do not understand what the Church's role is from God, we will abort in our mission, bring disdain to God's name, and relegate the lost to eternal fire since they will not be hearing the alarm to 'turn and live'. The Church alone proclaims the Gospel of life, so if we become diverted in our mission, God's grace is not heard. That's immensely sobering! We must take our godly role seriously, for it has eternal implications. God help us!
(Taken from: D. Martyn Lloyd-Jones, Darkness and Light: An Exposition of Ephesians 4:17-5:17. Baker Book House), pp. 313-328.