After all that had happened with Jesus, even after the resurrection when he was seeing people, visiting with people, even eating with them on the beach, he did give one last command to them all. That they are his last words, should be very telling to everyone.
In Matthew 5:13-16, it says that believers in Jesus, are to be salt and light. The world is often dark and just watching the news can give one a sense of foreboding at times. Life is precious, and quick. People need hope, and if its true what the Bible says about Jesus, it makes sense that He and others would want all to know about it.
Going to Matthew 28:16-20, you can see the actual commission by Jesus to the believers. They are to spread the hope and news of the Kingdom of God. Notice, he doesn't say to force it on anyone, at any time in history. There is to be no violence, just a sharing of the truth. People need to accept that not all will want this, nor accept it. You can't force it onto people even if you tried or wanted to. Its just a free gift of God, to those that want to respond. After that, you move on.
I am personally so thankful that someone ever shared the good news with me. Since that time, I have been learning and researching more, several decades now actually! I have only found reasons to reinforce what I already believe, and have put to the test many of the ideas that have been giving Christianity a "run for its money" so to speak. Its very refreshing to find, that no matter what, this simple truth seems to defend itself, without having to try. I mean, that truth would do that anyway. WHATEVER is true, will never have to really be fought for, it just is, except against things that are anti truth or even evil or at least immoral in the world.