Answer
The kingdom of God refers not, in the first instance, to a place but rather to God's reign, to his dynamic rule over his people. So, in the Old Testament, it is realized that God is the king. God is known as King. He reigns over all the earth. And yet, at the very same time, even though God was king, God's people knew he wasn't fully exercising his reign. So, the righteous suffered and the wicked seemed to be prospering. And so, there's this hope that grew up in the prophets that, in the last days, God would fully assert his reign, he would bring his kingdom, and at that time, he would vindicate his people, and he would judge his enemies. So, the kingdom of God is really where God asserts his kingship, his authority, his reign.
Answer by Dr. Stephen E. Witmer
Dr. Stephen E. Witmer is Adjunct Professor of New Testament at Gordon- Conwell Theological Seminary and Lead Pastor of Pepperell Christian Fellowship in Pepperell, MA.