Talking about the Kingdom of Heaven or the Kingdom of God, we understand that by having a Kingdom, there is a King in that Kingdom, and He is the ultimate ruler of that Kingdom.

The Kingdom is His domain and therefore, He is the supreme authority in His Kingdom.

This is the dominion of the King. Dominion is life. The opposite of life is death. The Gospel of the Kingdom of God is the Good News. John the Baptist preached about the Kingdom. Jesus preached about the Gospel of the Kingdom and Paul also preached the Gospel of the Kingdom of God.

Matthews 3:1-2 AMPC
(1) In those days there appeared John the Baptist, preaching in the Wilderness (Desert) of Judea
(2) And saying, Repent ([a]think differently; change your mind, regretting your sins and changing your conduct), for the kingdom of heaven is at hand.

Matthews 4:23 AMPC
(23) And He went about all Galilee, teaching in their synagogues and preaching the good news (Gospel) of the kingdom, and healing every disease and every weakness and infirmity among the people.

Acts 19:6-8 AMPC
(6) And as Paul laid his hands upon them, the Holy Spirit came on them; and they spoke in [foreign, unknown] tongues (languages) and prophesied.
(7) There were about twelve of them in all.
(8) And he went into the synagogue and for three months spoke boldly, persuading and arguing and pleading about the kingdom of God.

We see in these Scriptures when the Kingdom of God was preached, there was power. Hearing the Gospel, John the Baptist was saying: a man had to think differently, change his mind, regret his sins, and change his conduct.