To these He also presented Himself alive after His suffering, by many convincing proofs, appearing to them over a period of forty days and speaking of the things concerning the kingdom of God. Acts 1:3

This passage was one I read through a number of times before finally noticing the kingdom of God part. The fact that Jesus spent His final 40 days on earth with His disciples focused on the kingdom should tell you just how significant this is. If you read through the gospels you will also see that the good news of the kingdom of God was His central theme that He preached and taught.
But He said to them, “I must preach the kingdom of God to the other cities also, for I was sent for this purpose.” Luke 4:43

Paul continued the theme.

“And now, behold, I know that all of you, among whom I went about preaching the kingdom, will no longer see my face. Acts 20:25

There is a lot more concerning the kingdom but wanted to put these few scriptures out there as food for thought.