The Old Testament of the Bible is filled with prophecies foretelling the rise and fall of earthly kingdoms (Egypt, Syria, Babylon, Persia, Greece, Rome, etc.) and the coming of an eternal kingdom, which would be ruled by a future king, revealed in the person of Yeshua Hamashiach, Jesus the Christ.
In the New Testament, this eternal kingdom was proclaimed with boldness (by John the Baptist and Jesus of Nazareth). Many were warned that it was time to repent, for the kingdom was coming; it was at hand; it was near. This kingdom was demonstrated in power (through the works of expelling demons, healing of the sick, raising of the dead, and liberation of the oppressed). The kingdom grew exponentially in its citizenship due to many believing in and receiving the Lordship of its messiah and king.
This kingdom is not like all the other temporal, earthly kingdoms. Its origins do not derive from this evil world system that is in rebellion to God. This kingdom is a heavenly kingdom, and though it exists in the spiritual dimension, it is manifested in the hearts and lives of believers (the kingdom’s citizens).
The will, or agenda of this eternal, heavenly kingdom was revealed through Jesus, which is namely to destroy all the works of the devil – including sin and death – and to qualify its citizens to partake in an inheritance of eternal life and immortality through the gift of the glorious gospel. This will has been activated and accomplished through the death (and later resurrection) of its king, Jesus during his first coming.
At a time set aside by God the Father, this eternal kingdom (later described in Revelation as the Millennial Kingdom) will make a physical appearance during Jesus’s second coming, when it comes down from Heaven to rule on the earth for a thousand years (along with all of its citizens, the Saints of God). Then, after an intense battle with (and victory over) Satan during a war called Armageddon, the kingdom of God will continue its reign eternally in the new heaven and new earth once the present system of things is destroyed by fire at the Final Judgment.