One of the significant changes was a new understanding of the "Kingdom of God" or "Kingdom of heaven." I'm no 1st century Jewish scholar, so I'm doing the best I can with the tools/understanding that I have. It seems that if you were a 1st century Jewish person the concept of the kingdom of God or heaven was a strictly future concept. The kingdom was something that would come with the ruling messiah. Also, the kingdom would be an earthly or political kingdom - a here and now kingdom. When the messiah came there would be an earthly/political kingdom established where God would rule through the Jewish people. So until the messiah there was no kingdom; when the messiah came the kingdom would come in fulness.
Jesus presents a different vision of kingdom or "the" kingdom. As messiah, Jesus does inaugurate the kingdom of God. He brings a "here and now" kingdom to the Jews and the rest of humanity. Although, we have to recognize that there is a "there and then" element that is alongside the "here and now." While the kingdom is inaugurated, it would be consummated until the second coming of Christ. Practically, this means that the kingdom has been started and its effects have begun to be felt, but the full impact of the kingdom will not be experienced until the return of Christ. So we have a "now and not yet" sort of situation.
One very relevant passage is Matthew 10 (10:7). Here Jesus commissions (my word) His disciples to go into ministry healing, etc. Verse seven says this, "As you go, preach this message: 'The kingdom of heaven is near.' " They are to preach the message to the "lost sheep of Israel" (v. 6) (The identification of the "lost sheep of Israel" is worthy of a whole separate blog post - I just want to stick with a discussion on the kingdom). Two things come out of this that are interesting. First, the kingdom is "near." Second, healings, raising individuals from the dead, the driving out of demons, etc. are signs or evidence of the kingdom that is near. Reading through a Christocentric lens we see that Christ has brought the kingdom near in Himself. Through and because of His name the disciples are able to heal, etc and give tangible evidence (truth) of this kingdom. This would, I believe, have been revolutionary for a 1st century Jewish person.
What can be said about a kingdom that has been brought "near" or inaugurated? It has begun!! The new has come and the old has gone. We are being transformed. Transformation has begun, but we will not see full transformation until the final coming of Christ. This means something for history and culture. Yes - dark still exists, but the light is slowly chasing out the darkness - the victory is already won and the end of the story is known. Once the "Word has been made flesh" (John 1) we have seen the "glory" (John 1). Glory changes everything.
It must be acknowledged that the kingdom has already come . . . and come in Christ. No, it isn't what it will be, but it is surely better than what it was!
The implications for history and culture are obviously huge and more than I want to go into presently. This is where liberation theology, black theology, and environmental theology can make huge contributions. Jesus changed the trajectory of things for the 1st century Jewish person and for us in the 21st century.
It is curious to me as to why we don't live as if the kingdom has already come.