What does Mark 16:18 mean?

This verse is the least authentic in Mark 16. Where most of Mark 16:9–20 is accounted for elsewhere in the Bible, one facet of this verse is entirely unique.

Jesus promised the disciples that in the course of spreading the gospel, they would be able to "tread on serpents and scorpions" (Luke 10:19). Outside of a short illustration (Luke 11:12), scorpions are not mentioned again until Revelation which seems to speak of locust-like demons with the sting of scorpions (Revelation 9:1–11).

Similarly, the Bible doesn't recall the literal appearance of snakes except for Paul's run-in with a viper (Acts 28:1–6). While Paul was tending a fire, a viper, drawn by the heat, bit him on the hand. He shook the snake off into the flames and went about his business, surprising the locals who expected him to die.

Disciples did heal in the course of their evangelism. First, Peter and John healed a lame beggar (Acts 3:1–10). Soon after, the Holy Spirit becomes so present that apparently even people who crossed Peter's shadow were healed (Acts 5:12–16). Later, the Holy Spirit validated Paul's ministry by healing those who touched an apron or handkerchief that Paul had touched (Acts 19:11–12).

Nowhere, however, does the Bible record the disciples drinking poison and living. The corroborating account is from the apocryphal text "The Acts of John." The book says that the apostle John was challenged to drink poison; he did and lived. We have no way of knowing if this account is true, but that apocryphal work's many other flaws make everything in it subject to serious doubt.

What we can say is that this verse does not tell believers to handle snakes or drink poison. If the disciples survived serpents and poison, it was because the Holy Spirit was validating their message through miracles. But as Jesus told Satan, "You shall not put the Lord your God to the test" (Matthew 4:7).