I believe that scripture guides our beliefs and actions.
Here's your passage:
Mark 16:8,14-20
[Some of the earliest manuscripts do not include 16:9–20.]
Afterward he appeared to the eleven themselves as they were reclining at table, and he rebuked them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen. And he said to them, “Go into all the world and proclaim the gospel to the whole creation. Whoever believes and is baptized will be saved, but whoever does not believe will be condemned. And these signs will accompany those who believe: in my name they will cast out demons; they will speak in new tongues; they will pick up serpents with their hands; and if they drink any deadly poison, it will not hurt them; they will lay their hands on the sick, and they will recover.” So then the Lord Jesus, after he had spoken to them, was taken up into heaven and sat down at the right hand of God. And they went out and preached everywhere, while the Lord worked with them and confirmed the message by accompanying signs.]]
The inconsistency with the rest of scripture suggests a scribe put this in as a personal thought much later after the original author wrote it. Thank you, however, for providing your scriptural reasoning.
Looking at this passage, and taking it literally, one would have to teach that only if you get baptized can you be saved. All Christians would also be able to cast out demons, speak in tongues, pick up poisonous snakes without being harmed and drink all forms of poison without any harm. (Care to drink some anti-freeze?)
Ps91, do you do all those things with full assurance you will not be harmed? I think this passage should be held lightly, but you may hold it tightly.