But I am very open to scripture being presented that backs this up, without circular reasoning of course.
The term 'eternal' infers by implication a nature that has no beginning of existence. It was not created nor would it have came into existence by a process of evolving; thus it would be defined, at least in part, as having always existed.
The principle of the first cause argument requires the last effectual cause to have a cause, if not the effectual cause fails to establish it as the valid cause. So when you work the cause and effect back to the beginning, or the last known effect you will end with the expanse of celestial space in which all mass within this universe exists within.
You could say that space is the foundation upon which the world was built. However, you can't say that space and mass were brought into existence at the same time since space has to proceed mass since mass did not exist prior to the beginning and it requires space to in order to exist. I will use water as an example, before water could exist there would have to be two atoms of hydrogen and one atom of oxygen.
So we can reason that everything that exists within this universe had a beginning and the last known effect ends up as being the expanse of celestial space in the first cause argument. Now the question is what is effectual cause of space. Since everything known and observed to exist has an cause, then what is the cause produced the expanse? But remember, to be a valid effectual cause it must have a cause.
Now back to the term 'eternal', since nothing within this universe has always existed, which we conclude by the nature of the things in this world which reflects that living things within this world have a beginning and end of its life cycle. Hence, we conclude that any living entity that existed before this spherical expanse of celestial space was brought into existence would be outside of the time which is relative to the universe and that within.