"If you say you are a Christian, then act like one."
The assumption behind this aphorism is that a particular (written or unwritten) moral code defines, confines, or constrains the ontology of a Christian. The Christian's actions must line up with this set of pearl-strung laws or he/she does not qualify to be one. But Christianity is not moral code. It is a response to a moral code. Christianity revolves around the basic fact that Jesus Christ, not our own personal actions or our ability to live up to and emulate a pre-selected list of rules, is our righteousness. Those who "act like Christians" are, nine times out of ten, seeking to follow a law of righteousness that they have built for themselves. But this is not, in any sense, a submission to God's righteousness. Jesus says in the Sermon on the Mount that, "twisted as you are" you ought to come to the Father and expect good things from Him (in Luke Jesus modifies this teaching to disclose the receiving of the Holy Spirit). In other words: "I did not come to call the righteous, but sinners to repentance."
Pastors who engage in moralism in their sermons never take God's Holy Law seriously. They assume that if a person doesn't get drunk, sleep around, cheat their neighbor, or lie, he has sufficiently demonstrated enough good behavior to honestly call himself a Christian. Or positively, a person who writes encouraging emails to said pastor, attends church faithfully, participates in local ministries, and tithes generously is "acting" like a Christian. God's Holy Law will allow for none of this. According to the sacred writ in Romans, both the pious Jew and religious Gentile are forced to clap their mouths shut as God righteously damns them for their ungodliness and wickedness. They are unclean. Their good deeds and Christian actions are worthless in God's sight. They have aroused His wrath which burns to the grave below and cannot be quenched. They have sighted God's free gift of salvation and erected a triple-barrior of requirements in front of it. James says," Whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it." To this day, I have not met a man or woman who takes this verse seriously. Since it is most certainly true, the one embodiment of truth and knowledge that a Christian has to guage the autheniticity of his "Christianess" stands clearly and immutably against him. His flimsy structure of good deeds and partial fulfillments fall before one word. He has kept certain commands but has neglected others (love your neighbor as yourself? love your enemies? love God with your whole heart? Be perfect? Be holy as God is holy? Avoid so much as a hint of sexual immorality, greed, and impurity?) and thus he has kept no commands. He has broken all of them.
"If you say you are a Christian, then act like one" is a lawless statement. It is a call to take up that filthy pile of fishy-smelling menstrual rags and build up a ramp to Heaven. You may be certain that God will come down and scatter your pride.