For the 40 days following the resurrection, "He [Jesus] presented Himself to them with many convincing proofs that He was alive" - Acts 1:3a

And what was the primary focus of His instruction during this time?

He "spoke about the kingdom of God" - Acts 1:3b

The emphasis of His instruction was so strong, that it prompted the following question from His disciples:

“Lord, are you at this time going to restore the kingdom to Israel?

Jesus replied, 

“It is not for you to know times or seasons that the Father has fixed by His own authority. But you will receive power when the Holy Spirit comes upon you, and you will be My witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”

"Your kingdom come..."