ANSWER: it was initially established on the day of Pentecost. When the Son of God "had overcome the sharpness of death, he opened the kingdom of heaven to all believers." The kingdom of God is the same as the kingdom of heaven. It is invisible. All who submit to Christ are it's subjects. However, in some of Christ's parables the kingdom of heaven is apparently identified with the visible church containing the good and the bad, the fares and the wheat, the wise and the foolish virgins. It is spoke of as a drag-net enclosing fishes worthless and good.
— Steele's Answers p. 81.