What does Mark 16:17 mean?

Mark 16:9–20 is not found in the most reliable manuscripts, but most of the text is included in other books. Some phrases, however, are only corroborated obliquely. Although Jesus had endowed the disciples with the ability to cast out demons in the past (Mark 6:13), and they will cast out demons and speak in new languages in the future, we have no other evidence He said these words to them.

In the beginning years of the church, casting out demons was a common miracle that identified the speaker as a follower of Jesus (Acts 5:16; 8:7; 19:12). In one case, Paul cast out a demon just because it was irritating him (Acts 16:18).

There's no record that the disciples had spoken in languages they didn't know before this point, but on Pentecost, when the Holy Spirit comes to them, they will (Acts 2:1–13). They will be in Jerusalem, along with Jews from all over the Roman Empire. The people will hear the disciples explaining the gospel in the languages of the Parthians, Medes, Mesopotamians, Egyptians, Libyans, and Romans, among others. This is not the gibberish that some call the "gift of tongues." It is real languages that the speakers honestly don't know. Jesus had already promised the disciples would receive guidance on what to say when brought before hostile rulers (Mark 13:9–11), now they learn the Holy Spirit will also show them how to say it.

These powers, in addition to gifts of healing and other miracles, were common in the early church age. As with many of God's prophets, He empowered them to perform miracles so that their words would be validated. With the finalization of the canon of books determined to be inspired, we do not need miracles to affirm Jesus' words. God does still work directly in people's lives through miracles, but it is not something we should expect.