As with many words and phrases in the Bible, kingdom of God has more than one meaning. It’s obvious in many passages that “kingdom of God” (and the equivalent “kingdom of heaven”: used only by Matthew) in the New Testament refer to a spiritual reality, as opposed to the physical and “institutional” messianic kingdom to come. This was foreshadowed in the Old Testament in the motif of changed “hearts” that served and followed God: especially in Jeremiah:
Jeremiah 31:33 (RSV) But this is the covenant which I will make with the house of Israel after those days, says the LORD: I will put my law within them, and I will write it upon their hearts; and I will be their God, and they shall be my people.
Jeremiah 32:40 I will make with them an everlasting covenant, that I will not turn away from doing good to them; and I will put the fear of me in their hearts, that they may not turn from me.
Also, being inhabited by God’s “spirit” in the Old Testament was a precursor to Pentecost and all Christians being indwelt by the Holy Spirit (essentially, being in the kingdom of God; regenerated, justified, sanctified, etc.):
Numbers 11:29 But Moses said to him, “Are you jealous for my sake? Would that all the LORD’s people were prophets, that the LORD would put his spirit upon them!”
Psalm 51:11 Cast me not away from thy presence, and take not thy holy Spirit from me.
Isaiah 42:1 Behold my servant, whom I uphold, my chosen, in whom my soul delights; I have put my Spirit upon him, he will bring forth justice to the nations.
Isaiah 44:3 I will pour my Spirit upon your descendants, and my blessing on your offspring.
Isaiah 59:21 “And as for me, this is my covenant with them, says the LORD: my spirit which is upon you, and my words which I have put in your mouth, . . .” (cf. 63:11)
Ezekiel 36:27 And I will put my spirit within you, and cause you to walk in my statutes and be careful to observe my ordinances. (cf. 37:14; 39:29)
Joel 2:28 “And it shall come to pass afterward, that I will pour out my spirit on all flesh; . . .” (cf. 2:29; Hag 2:5)
Zechariah 7:12 . . . the words which the LORD of hosts had sent by his Spirit through the former prophets. . . . (cf. 4:6)
Here are some of Jesus’ many uses of these phrases in a strictly spiritual sense:
Matthew 5:3 “Blessed are the poor in spirit, for theirs is the kingdom of heaven.” (cf. Lk 6:20)
Matthew 11:12 “From the days of John the Baptist until now the kingdom of heaven has suffered violence, and men of violence take it by force.”
Matthew 12:28 “But if it is by the Spirit of God that I cast out demons, then the kingdom of God has come upon you.”
Matthew 19:12 “For there are eunuchs who have been so from birth, and there are eunuchs who have been made eunuchs by men, and there are eunuchs who have made themselves eunuchs for the sake of the kingdom of heaven. . . .”
Matthew 19:24 “Again I tell you, it is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God.” [this is the famous “rich young ruler” incident. Jesus appears to define the term as “eternal life” (19:16, 29), or spiritual “life” (19:17), or “treasure in heaven” (19:21), or being “saved” (19:25) ]
Mark 12:34 And when Jesus saw that he answered wisely, he said to him, “You are not far from the kingdom of God.” . . .
Luke 7:28 “I tell you, among those born of women none is greater than John; yet he who is least in the kingdom of God is greater than he.”
Luke 10:9 “heal the sick in it and say to them, `The kingdom of God has come near to you.'”
Luke 11:20 “But if it is by the finger of God that I cast out demons, then the kingdom of God has come upon you.”
Photo credit: Jesus Christ with the children / Let the little Children come unto Me / Suffer the Children, by Carl Bloch (1834-1890) [public domain / Wikimedia Commons]