Revelation 11:15-17

15  And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.

16  And the four and twenty elders, which sat before God on their seats, fell upon their faces, and worshipped God,

17  Saying, We give thee thanks, O Lord God Almighty, which art, and wast, and art to come; because thou hast taken to thee thy great power, and hast reigned.

After the two prophets are raised from the dead, the angel blows the last trumpet of the seven. This trumpet brings in the seven vial judgments that will occur in these last 3 ½ years. They are the last judgments that will cleanse the earth before Jesus actually comes to set up His kingdom. It is also the last woe for mankind. When this trumpet is blown, the kingdoms of the world are given to Jesus. It will be time for Jesus to take the kingdom of heaven to earth but first rewards must be given to His saints in heaven and the earth must be cleansed of all evil doers.

The authority to rule on earth was won at the cross but the timing to take possession was set by the Father and delayed so that many people could come to His Son for salvation. The passage says that He has taken His great power and has reigned. It implies that up to this point in time He had the power but He had not taken His power to reign on the earth.