How do these elders address God?

They call him Lord, and then attach three more terms to describe who the Lord is: 1. God; 2. the Almighty; 3. he who is and who was.

• The term Lord is the Greek word for master or owner. At the same time it was used in the Greek translation of the OT to translate the term Yahweh, the Creator who established a bond of love with man and maintained it (through Jesus Christ) even after his fall into sin. 

• The term God catches his divinity, his otherness, his being different from any creature. 

• The word almighty describes his power, a power great enough not only to create a world but also great enough to be crowned King of the world before whom even privileged elders rightly fall on their faces. 

• The last term, who is and who was echoes the tripartite formulation of Revelation 1:4 and Revelation 4:8: who is and who was and who is to come. Concerning the first two members of this tripartite title, see commentary on Revelation 1:4. As to the third component of this threesome, in Revelation 11:17 it is missing. The reason is as the heavenly voices had just proclaimed: Jesus Christ has come, and so has become the king of the world. 

Together this series of names these twenty-four elders use gives their estimation of who the God is in whose presence they dwell and whose works they may witness.