He said to them, “Go into all the world and preach the gospel to all 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 drive out demons; they will speak in new tongues; they will pick up snakes with their hands; and when they drink deadly poison, it will not hurt them at all; they will place their hands on sick people, and they will get well.” (Mark 16:15-18 NIV)
Go into all the world
That means go where the Holy Spirit sends us, leads us and confirms to us where we should be. It probably means going to someplace else. That's what all of his guys did.
We are to follow their example. That is our job.
Preach the gospel
That means proclaim the Good News (gospel, a religious term) about salvation and new life through Ye-HO-shu-a Ha-ma-SHEE-ach (Jesus the Christ or Messiah, King of Israel), and everything else the Bible teaches.
Whoever believes and is baptized (by immersion, like Jesus was) will be saved
Whoever does not believe will be condemned
Our own belief is required.
Our own baptism is part of the job. It is not necessary for salvation, according to this text, but we aren't going to get much done without it. (Other places in the Bible say it is required for forgiveness of sins. Acts 2:38 is one of those places.)
These signs will accompany those who believe
In other words, if the following things are not happening, then logically we do not believe.
And if we do not believe then we are not saved.
Consequently, it is crucial we make an effort to do these things.
If we attempt to do these things, we will see them happen. If we don't, we won't.
If we see them happen, our belief will increase and we will become more confident.
If we become more confident, we will want to do these things more.
If we do these things more, we will bear much fruit. Fruit that will last.
In my (Yehoshua's, i.e. Jesus') name they will drive out demons
Jesus did it. His guys did it. We must do it.
Getting rid of demons is job one (1). It is listed first.
If infiltrators are allowed to remain embedded and entrenched, they are going to interfere with our work and we will accomplish nothing of eternal value. We will not be able to bear good fruit, fruit that will last.
Demons can be driven out with a single word (Matthew 8:32). We have the authority to do it. We must exercise that authority and power.
We must learn to value the man or woman who was made in the image of God and distinguish between them and the bondages and infirmities that plague and imprison them. Demons can be the cause of such things. Demons are not people. Having a demon, or demons, is not shameful. We are all susceptible to them.
Demons can affect our behaviors and beliefs. Getting rid of demons will give us clear minds and perspectives.
Jesus can fix all of our problems.
And he will do it through his people during this period of time. He is sitting next to the Father and intercedes for us as we do our work.
They will speak in new tongues
In other words, we will do many miraculous things when following Jesus through the Holy Spirit.
They will pick up snakes with their hands
In other words, we will not be afraid. God will be with us to protect us and to demonstrate his authority and power through us.
When they drink deadly poison, it will not hurt them at all
The opposition will be powerless to oppose us. They can try, but they will fail.
They will place their hands on sick people, and they will get well
God will miraculously change people's lives through us, and it will cause those people to listen to our message and perhaps become believers themselves.
Jesus heals all sicknesses and diseases.
What part of "all" do you not understand?
Doing the above things is our job.
Nowhere in the Bible does Jesus say, "Go to church." It's not there.
Justifying our own behavior does not change that fact.
Some Bible translations convey doubt about the origins of this last part of Mark. In my opinion, the Darkness has done what it can to keep important and clear instructions from people by interfering with the stewardship of important and clear sections of God's Word.
But, God is sovereign and infinitely greater than any potential interference with His purposes.
All glory to Him.