Q. Aren't biblically unsound and even cultic groups doing good in the world by promoting standards of decency and morality? Does Luke 9:49-50 say we shouldn't forbid them?
A. Let me begin by asking my own questions: What is the commission Christ gave to His disciples? Is it to promote decency and morality? Or is it to preach the Gospel message that Jesus died to save sinners who will trust Him as their Savior?
In Luke 9:49-50, we read, "John answered, 'Master, we saw someone casting out demons in your name, and we forbade him, because he doesn't follow with us.' Jesus said to him, 'Don't forbid him, for he who is not against us is for us.'" Does this mean that we shouldn't ever criticize or warn people against any other teachers, preachers, churches, or ministries however unbiblical they may be?
We should notice that Christ says, "He who is not against us is for us." He could not say this if the man were preaching or doing anything contrary to the teachings of Christ. Later, Jesus teaches, "He that is not with me is against me. He who doesn't gather with me scatters" (Luke 11:23). This further strengthens the teaching: To be for Jesus, we must not be against Him and we must be with him—gathering with Him. The question to ask about a group, then, is, does it fit this standard? Is the teacher, church, or group for or against the work of Jesus Christ?
The followers of Herbert W. Armstrong, the Jehovah's Witnesses, the Mormons, and many other fringe groups add their own requirements (keeping the law, observing a Sabbath day, adhering to dietary restrictions, following "another revelation of God," and so forth) to the way of salvation taught by Jesus Christ and his apostles. For that matter, so do many churches that are not considered on the fringe. Some churches also preach a distortion of God: Oneness Pentecostalism, God is a family we can enter rather than being a Trinity, the Holy Spirit is an impersonal force, open theism, Jesus is not fully God, Jesus did not fulfill the law, etc. These teachings ultimately necessitate the discarding of the true Gospel and the preaching of another gospel. Would Jesus have us stand aside and say nothing while these groups give the world a wrong impression of Him? No.
Certainly, it's true that some groups, many in fact, promote morality. But promoting morality is not the standard God wants for those who take up the name of Jesus. Even the Jewish leaders of Jesus' day promoted morality, but God destroyed their system (and many of them) in AD 70. The promotion, and even the enforcement, of decency and morality are the domain of the secular authorities (Romans 13). When we Christians promote these things, we give our hearers the false impression that they are doing well in God's sight by simply being moral. But that's a lie and a very harmful one, too.
Promoting morality over the Gospel reminds me of giving someone who has operable cancer a pain killer and not telling him he needs to have the cancer cut out. He can go on for a while thinking he's okay, until the cancer kills him. Teaching people to follow a regimen of morality is, at best, palliative only. It can make some people feel good about themselves. But it doesn't cure the problem. For other people, moral teaching only makes them feel worse. I suppose these people are more honest with themselves. They know they can't live up to these standards. But if they're not told the real solution and believe it, they, too, will die in their sins. This is why those who preach morality, the law, political action as a solution for ills that only the Gospel can solve, a Christ who is a mere example rather than a true Savior, a God who accepts worship of false gods as if it were worship of Him, a soothing message that says sinners can still be saved after death, an atonement that hasn't completely taken away all of our sins and put us under the New Covenant that has no condemnation, or any other way of salvation but Christ alone must be exposed as preaching a false Gospel. They're keeping people from the true Gospel, and, thus, they are working against Christ.
I have no doubt that the Judaizers whom Paul was criticizing in his letter to the Galatians promoted standards based on the Ten Commandments and other aspects of the law. Because they tried to add these other requirements to the way of salvation, however, Paul said they were preaching a different and perverted gospel (Galatians 1:6-9). He did not consider they were working for Christ nor that they were doing good in the world, and he did not believe that he should just leave them alone. Paul warned the Galatians against them (and, thus, also warned us against such teachings) and pronounced a curse on them.
Today we live in a time of great apostasy when large numbers of people teach false gospels in the name of Christ. We have a duty to expose these people, warning our brethren against their dangerous doctrines, and possibly even waking up the false teachers themselves so they can be plucked from the fire.