These verses also support the primary importance of believing. Those who believe, not just the Eleven, would continue to perform supernatural acts. Throughout Scripture such "signs" always signified that something of supernatural origin was happening, and they authenticated the message that the witness bore (cf. Mark 16:20). [source][source][source]
"The signs authenticated the faith the early believers proclaimed, not the personal faith that any one of them exercised." [1][source]
The Twelve had already cast out demons and healed people in Jesus" name ( Mark 6:7; Mark 6:12-13). They would continue to have these abilities (cf. Acts 5:16; Acts 8:7; Acts 16:18; Acts 19:12; Acts 28:8). This is the only reference to the disciples speaking in tongues (i.e, languages) in the Gospels (cf. Acts 2:4; Acts 10:46; Acts 19:6; 1 Corinthians 12:10; 1 Corinthians 12:28; 1 Corinthians 12:30; 1 Corinthians 13:1; 1 Corinthians 14:2; 1 Corinthians 14:18-19). There is no textual basis for distinguishing the unlearned languages spoken in Acts from the gibberish that some claim the epistles refer to. Tongues in the New Testament were evidently always languages. [2] Immunity from the bite of poisonous snakes was another privilege the disciples would enjoy (cf. Acts 28:3-6). There are no examples of disciples drinking something deadly and surviving in the Book of Acts. [source][source][source]
Jesus did not say how long the disciples would be able to do these things. Previous periods of miracle-working had all been fairly short (cf. Exodus 7-14; 1 Kings 17 - 2 Kings 10). Therefore that is what the disciples could expect (cf. 1 Corinthians 13:8). Church history has confirmed that the period of miracle-working that existed in the first century passed away about the same time as the completion of the New Testament canon (cf. 2 Corinthians 12:12; Hebrews 2:3-4). Some Christians claim these promises are valid today, for example the snake-handling and poison-drinking sects of Appalachia. However these were mainly promises of divine protection when the disciples" persecutors would compel them to do these things. [source][source][source]
God still sometimes convinces people of the truth of the gospel or confirms the truth of His Word to people with supernatural experiences. Nevertheless these are not the same experiences as what Jesus promised here. Some of the early Christians could perform miracles whenever they wanted to do so in God"s will (e.g, Acts 3:6; Acts 16:18). That is not the case today, though God still performs miracles today. [source][source][source]