I am not sure about the "religious definition of eternity" inasmuch as there are so many religions. I prefer to synthesized ideas about God and eternity and to reach my own conclusion.
For me, God is eternal because He created all creatures, including Angels. God's eternity must be the absence of time, or no beginning and no ending. Because it is impossible to know the mind or God, we can only speculate. Humans are handicapped by language, a delimiting structure based on culture within a time bound world. The creator has no such handicap, no language restrictions, no grammar to negotiate, and no culture to derive meaning. Outside of time, God's view is beyond our understanding. We can however discuss how God's creatures confront time, a concept related to eternity.
When God created the angels, He created a time line in heaven. For each angel, time is different, but because they were created to be with God for all of eternity, time is of little importance in heaven. Then, when God created the universe, He created a time line for mortal creatures. For humans, time is extremely important, it determines :faint:life in an evolving and changing world. Because we can only relate to time, we have a very limited understanding of eternity. If time is a product of God's creation, eternity is beyond understanding.
Many ideas about God and eternity have been proposed. My favorite idea is that God knows everything that happens before it happens. Well, then why can't God eliminate bad outcomes? I think you have to infer that God has moral character. If it where not so, you could infer that we are all robots without purpose, or we just carry out the program. At this junction, religion becomes relevant, or even philosophy, or any discipline that discusses the meaning of life, and those meanings in the context of human experience.