If you knew you only had a short time to live, what would be your final conversation with your family? Wouldn't you communicate what you considered to be most important? Surely Jesus did the same.
I've been profoundly challenged in thinking about this (thanks, Neil Gamble!) In the few days Jesus had with his disciples after his resurrection and before his ascension, he talked with them about his Kingdom; several individual conversations are also recorded. However, the accounts of what he instructed the disciples as a group together are remarkably similar in all four gospels:
Jesus came and told his disciples, “I have been given all authority in heaven and on earth. Therefore, go and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit. Teach these new disciples to obey all the commands I have given you. And be sure of this: I am with you always, even to the end of the age." (28:18-20)
And then he told them, “Go into all the world and preach the Good News to everyone. Anyone who believes and is baptized will be saved. But anyone who refuses to believe will be condemned. These miraculous signs will accompany those who believe: They will cast out demons in my name, and they will speak in new languages. They will be able to handle snakes with safety, and if they drink anything poisonous, it won’t hurt them. They will be able to place their hands on the sick, and they will be healed." (16:15-18)
Then he said, “When I was with you before, I told you that everything written about me in the law of Moses and the prophets and in the Psalms must be fulfilled.” Then he opened their minds to understand the Scriptures. And he said, “Yes, it was written long ago that the Messiah would suffer and die and rise from the dead on the third day. It was also written that this message would be proclaimed in the authority of his name to all the nations, beginning in Jerusalem: ‘There is forgiveness of sins for all who repent.’ You are witnesses of all these things.
"And now I will send the Holy Spirit, just as my Father promised. But stay here in the city until the Holy Spirit comes and fills you with power from heaven." (24:44-49)
“Peace be with you. As the Father has sent me, so I am sending you.” Then he breathed on them and said, “Receive the Holy Spirit. If you forgive anyone’s sins, they are forgiven. If you do not forgive them, they are not forgiven.” (20:21-23)
“Do not leave Jerusalem until the Father sends you the gift he promised, as I told you before. John baptized with water, but in just a few days you will be baptized with the Holy Spirit." (1:5)