saying "We give Thee thanks,
All our worship of God should begin with THANKS.
O Lord God, the Almighty,
This is the fullest, most majestic title of God. There is no doubt to WHOM this thanksgiving is directed. Recall that Satan wanted Jesus to WORSHIP him in Luke 4:7. In return, Satan would give Jesus the kingdoms of the earth (now belongs to Christ, verse 15). How wrong Satan was!
who ART and who WAST,
King James Version Bible adds 'who art to come'. BUT He has come and
because Thou hast taken Thy great POWER
This is the same POWER that was promised to us in Acts 1:6-11.
and hast begun to REIGN.
There is a time (we are living in it now), where Satan is permitted to REIGN or be ruler over the earth. During this period, Jesus' wish is that none should perish (2 Peter 3:9), but with the seventh trumpet (third woe), the time for repentence is over. Christ has begun to REIGN. The STONE of Daniel 2:31-35 and Daniel 2:40-44-45 is in the process of crushing kingdoms. It takes GREAT POWER to accomplish this (Luke 12:5). Jesus prayed that this POWER to REIGN would be given to His disciples, John 17:6-18-19.
And aside note: GOD, THE FATHER and HIS SON, the CHRIST, have always reigned. After all, they created everything (Genesis 1-2). On the other hand, each individual's existence begins at conception. That would be the earliest time, that we would/could know that GOD is reigning. Your individual choices throughout your physical life will determine if you are part of the "bride (righteous acts of the Saints)". There is only an eternal REIGNING with God and His Son after the marriage takes place (Revelation 19:1-8-10 and Revelation 21:1-8).