The kingdom of heaven is the rule of the heavens in the Person of the Son of Man.
It was preached by John the Baptist (Matt. 3:2); by our Lord Himself in the following chapter, verse 17. Later we find it preached by the apostles in chapter 10, verse 7.
When Christ as the promised King and Messiah to Israel is rejected, it becomes the "kingdom in mystery." (Matt. 13:11.) That is, Christ is not openly reigning, though faith is assured that God is behind the scenes moving all the scenes which He is behind. (Eph. 1:19-23; 1 Peter 3:22.)
Wheresoever the seed of the gospel has been sown and men have professed Christianity, there we have what Scripture speaks of as "The kingdom of heaven." It is the sphere of Christian profession on earth.