Some people warn against what they call "over-realized eschatology," which is a fancy way of saying, "You expect to see more of God's power and justice in this life than what is realistic." "The Kingdom is now and not yet," we say. But for some reason, we emphasize the not yet part. I assume we emphasize the not yet part because we are afraid of disappointment and lack faith. After all, what is the point of "good news" if we are supposed to follow it up with a severe disclaimer?
In Matthew 16, Jesus told His disciples that "some of you standing here will not taste death until they see 'the Son of Man coming in His Kingdom.'" (Matthew 16:28). Shortly after that statement, the disciples witness the transfiguration and see Jesus glorified. This event made them realize they were further along in history than they thought. The Kingdom of God was appearing right before their eyes. Despite this experience, when the disciples descended from the mountain, they encountered an evil spirit that they failed to cast out. This failure exasperated Jesus and confused the disciples.
"Why couldn't we cast it out?" Asked the disciples.
"Because of your lack of faith," Jesus replied. "I'm telling you the truth: if you have the faith of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you." Matthew 17:19-20
The problem the disciples had was an under-realized eschatology. I'm not sure that there is anything we could properly call an "over-realized eschatology." The Kingdom is here, or it is not here. The Gospel clearly states that it is here. When Jesus "preached the Gospel" in Mark 1:15, Mark clearly states that Jesus proclaimed, "the Kingdom of God is arriving."
Yes, there is more of the Kingdom to come. Every tear needs to be wiped away. All sickness and death needs to be defeated. All evil must be eradicated. We still pray for the Kingdom of God to come on earth as it is in Heaven. But when it comes to prayer, action, and our expectation of what God will do, we need to remind ourselves that the Kingdom has already come. After all, a crucial part of how God's rule and reign gets implemented on earth is via God's people. If we don't believe and behave as though the Kingdom of God is here, most of the world will never see it. It is vital for the world that the Church gets a better hold of the truth that the Kingdom of God has already come and is here now.