I have two things to say Alan;Jesus Opens the Meaning of Scriptures:
44; "And He said unto them, These are the Words which I Spake unto you, while I was yet with you, that all things must be Fulfilled, which were Written in the Law of Moses, and in the Prophets, and in the Psalms, concerning Me.
45; "Then Opened He their understanding, (SO THAT THOSE DISCIPLES NOW "BELIEVED" and) that they might understand the Scriptures,
46; "And said unto them, Thus it is Written, and thus it behoved Christ to Suffer, and to Rise from the Dead the Third Day:
47; "And that Repentance and Remission of sins should be Preached in His Name among all Nations, beginning at Jerusalem."
Then, notice how that after Jesus Opened the understanding of His 11 Disciples, they are the ones whom He Commissioned, as His first church assembly He was Building, for them, now that they believed, that BY THEM: "Repentance and Remission of sins should be Preached in His Name among all Nations, beginning at Jerusalem" and He described the Instantaneous Miraculous Gifts He would Give those of His 11 Disciples who now believe.
The "unbelief" and "belief"
correctly associated with the Disciples is the key to Mark 16.
Jesus Appears to Two Disciples:
(Luke 24:13–35)
Mark 16:12; "After this, Jesus Appeared in a different form to two of them as they walked along in the country.
13; "And they went back and reported it to the rest, but they did not believe them either.
There we have to mention again of "they did not believe", meaning, "that all things must be Fulfilled, which were Written in the Law of Moses, and in the Prophets, and in the Psalms, concerning Me."
The Great Commission:
(Matthew 28:16–20)
Then, "unbelief" is exactly what Jesus rebuked the Eleven about:
14; "Later, as they were eating, Jesus Appeared to the Eleven and rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen Him after He had risen."
Talking to His Disciples Jesus Commissioned them, as a church;
15; "And He said to them, “Go into all the World and Preach the Gospel to every creature.
But, then when this next verse speaks about "Whoever believes", it is a separate parenthetical phase placed in between all the other ones, where Jesus is referring to the people who the Disciples will be Preaching to and who would "believe" "the Gospel" and will be baptized as a Fulfillment of the same Commission they would be Preaching to those Saved souls:
16; "Whoever believes and is baptized will be Saved, but whoever does not believe will be Condemned."
So, Jesus' use of the word "believes" there in that parenthetical phase, it is referring to those who would be Saved under their Preaching.
BUT THEN, JESUS REFERS BACK TO THE DISCIPLES, WHO HE HAD JUST REBUKED FOR THEIR "UNBELIEF" next in verse 17 and the Instantaneous Miraculous Gifts He is saying these Disciples will have, doesn't connect or relate to those in verse 16 who will "believe" and be Saved.
Jesus is now Addressing the Disciples again, directly, AND IT IS TO THEM AND NOT EVERYONE WHO "BELIEVES" FOR SALVATION THAT JESUS BISTOWS THESE GIFTS HE TALKS ABOUT.
17; "And these Signs will accompany those who believe: In My Name they will drive out Demons; they will speak in new tongues;
See? Jesus is saying that, "these Signs will accompany" "THOSE" of you Disciples "WHO" NOW "BELIEVE" "that all things must be Fulfilled, which were Written in the Law of Moses, and in the Prophets, and in the Psalms, concerning Me"?
So, the Disciples who now had their understanding Opened and "believe" are the "they" and "them" underlined here that Jesus is referring to (and not everyone who gets Saved)
Again, not all "believers", but those Disciples who now "believe" are the ones Jesus is talking about when He says:
18; "they will pick up snakes with their hands, and if they drink any deadly poison, it will not harm them; they will lay their hands on the sick, and they will be made well.”
Pr 14:8 The wisdom of the prudent is to understand his way: but the folly of fools is deceit.
Not his words but his way.
Isa 28:9 Whom shall he teach knowledge? and whom shall he make to understand doctrine? [them that are] weaned from the milk, [and] drawn from the breasts.