Is casting out demons in Jesus’ name a sign of a true believer?

Mark 16:17 promises that such signs would go with believers. It does not say that every believer would do them, or that no one else could. ‘Signs will follow believers’ is not the same as ‘whoever does a sign is a believer’. The signs were given to the church as a whole, to confirm the gospel as it went out: the Lord was “confirming the word with signs following” (Mark 16:20). Acts shows it happening, as when Paul drove out a spirit “in the name of Jesus Christ” (Acts 16:18).

The man in Mark 9:38–40 was not an unbeliever. He did not belong to the Twelve (“he followeth not us”), but he acted in Jesus’ name, and Jesus counts him as on their side: no one who does a miracle in his name “can lightly speak evil of me”. Barnes suggests he may have been a disciple of John the Baptist or one of the seventy. So he fits Mark 16:17 rather than contradicting it.

Matthew 7:21–23 gives the other half of the truth: outward works, however striking, are not the test of a true disciple. Jesus does not discuss whether the works these people claim were real; he says that he never knew them, because they did not do his Father’s will. Judas was among the Twelve to whom Jesus gave power over unclean spirits (Matthew 10:1–4), and Scripture warns that false prophets can show signs (Deuteronomy 13:1–3; Matthew 24:24). The test Jesus gives is fruit: “by their fruits ye shall know them” (Matthew 7:20).

Signs for the first generation

Many Christians hold that the signs of Mark 16:17–18 were given chiefly to the apostles and the first believers, to confirm the gospel at its start (compare Hebrews 2:3–4). Others believe such gifts continue today. On either view the promise describes what would go with the church’s preaching; it is not a test for telling a true believer from a false one.

Claimed or real works

Some think the people in Matthew 7:22 really had done such works, as Judas had. Others think the works were only claimed, or done by another power. The sons of Sceva tried to use Jesus’ name as a spell and failed (Acts 19:13–16). Either way the point stands: works done in his name do not save anyone whom he does not know.

Mark 16:17 and 16:20 belong to the longer ending of Mark (16:9–20), which the two oldest complete Greek manuscripts, Codex Sinaiticus and Codex Vaticanus, lack. Many scholars, evangelicals among them, doubt that Mark wrote it; others defend it. It was known early: Irenaeus, writing about 180, quotes Mark 16:19 as coming ‘towards the conclusion of his Gospel’. The answer here does not depend on settling that question.