What does Mark 9:39 mean?

There are people in the world who admire Jesus as a good man and a wise teacher. They may even try to live their lives according to what He taught. It's unfortunately true that even those people, if they do not recognize their sin and allow Him to pay the cost for them, are not saved. But Jesus says don't discourage them. Let them continue to be inspired by Him. Certainly, teach them the truth about salvation, but don't minimize their good works (Acts 18:24–28). Works don't "count" toward salvation, but any God-honoring and God-obeying effort to do good in Jesus' name glorifies Him.

Both Moses and Paul understood this. When Joshua pleaded with Moses to control a group of unregulated prophets, Moses told Joshua he had no jealousy for other men whom the Holy Spirit empowered (Numbers 11:26–29). Paul went even further, validating the ministry of preachers who taught about Christ out of envy and rivalry. Paul had more concern for the spread of the gospel than his own reputation (Philippians 1:12–18).

Scripture will explain that not everyone who casts out demons in His name is saved; another sign is doing God's will (Matthew 7:21–23). Sometimes the demons will not respond, even to Jesus' name (Acts 19:13–16). And sometimes even those who follow Christ on the outside don't have the faith to cast out demons in His name (Matthew 17:14–20). Other times, a token that is carried by a Christ-follower will drive the demons away (Acts 19:11–12). The vagaries show that the power to exorcise demons comes from God alone and only in His timing and will. If someone casts out a demon, they are doing God's work, but that doesn't mean they belong to God.