In Jesus' words and statements about the kingdom very different aspects are addressed, especially in his Kingdom parables. Jesus points to the small beginnings (mustard seed, leaven), to the value or preciousness of the kingdom (treasure in the field, precious pearl), to the way of acceptance (seed on four types of soil), to opposition (weeds in the field) and much more.
But what for him the kingdom really is, especially from texts like Matthew 12:28 "But if I cast out demons by the Spirit of God, then the kingdom of God has come to you" and Luke 17:21 "For behold, the kingdom of God is in the midst of you".
Here the answer of Jesus to the question about the kingdom is almost obvious. Through him, through his presence and his actions, God came and now acts here, in the midst of history. Therefore the time is fulfilled (Mark 1:15), the kingdom has come, in Himself, in His person. He himself is the center of the message, in him we see both the inconspicuousness of the mustard seed and the preciousness of the treasure in the field.
But since his earthly course, which led to Golgotha, was not yet complete, he often spoke only veiled of his person. But after his ascension his disciples clearly and unmistakably put him in the center: the gospel of Christ. In fact, it is no other message than the one Christ proclaimed in his Gospel of the kingdom.
He had come to the midst of them, in the name and on behalf of the Father, for Jesus always speaks as a son; even as an empowered Lord he always remains the son! His unique relationship to the Father always stands behind all his words and teachings, behind all his actions.
Proclamation of the Kingdom of God is proclamation of Christ, proclamation of His mystery as Son, proclamation of the presence of God in the doing and being of Jesus. And that is why he is also the stone at which the spirits divide: The cornerstone upon which one builds, or the stone upon which one stumbles.
But everyone who proclaims Christ as Son and Lord proclaims the Kingdom of God in the same way; and Jehovah's Witnesses also proclaim the Kingdom of God only when and as long as they proclaim Christ as Son and Lord, place Him in the center of their message. Do they?