Forever seems like such a reasonable sort of thing sometimes; especially when our finite lifetimes seem to stretch out before us, on and on without end. From this day on. Till death do us part, forever! Eternity! etc!
But forever is a tricky thing for us mortals; of itself, forever is only ever going to be time without end. Not really what we’re after. Not convinced? Think of the fictitious lady from Grecian mythology, Aurora, who begged Zeus to make her husband Tithonous immortal. Her wish was granted, but soon Aurora recognised her horrible mistake. She had pleaded for immortality, but not for eternal youth. (In the story) Tithonous didn’t die, but he got very decrepit! Instead of Living, he was simply Existing; highly unpleasant!
Many people make this mistake about God's offer of eternal life. They think, "Why would I want more of this?". That's very understandable and reasonable, why should they?
A real-life Greek called Plato, grasped the limitations of the prevailing view of time, and defined a new word having important new dimensions of meaning. Plato’s word was Aionios: Eternal, Everlasting, Without Beginning or End. Later on the Greek speaking writers of New Testament picked up on this new word. Not because the Greeks didn’t have words for time, they did; eg. chronos and kairos; but this new word was able to provide a completely new way of thinking about Jehovah God, about Eternity, and about Jehovah’s interaction with His creation.
Importantly, aionios goes far beyond simply representing the passing of human years; it has to do with being outside of time itself. Actually it’s even more than that; it’s really about God’s possession and Lordship of time. We know that time is part of creation and something that we live in, but it's very tricky to pin down precisely, and doesn't even behave the way we expect it should. All the laws of kinematics (physics of motion) which are symmetric and reversable run perfectly well either forwards or backwards, up or down, left or right. Except for time, which simply does not behave the way that the other physical attributes (such as distance) do. Contrary to expectations, it will only run forwards, for humans anyway. The astronomer Arthur Eddington even coined a phrase, "the arrow of time"; to help us gain a handhold on this strange phenomenon.
So when Plato conceived of aionios, he was thinking in particular about the attributes of time that only God had control over. He proposed that time was something like a moving picture. That it was travelling along; and God who was standing outside the picture, could see time in its entirety, at a glance. Aionios is not just, forever; that is, time having no end; amongst other things, it includes being independent from time, and it is the domain of God Himself.
We enter into this domain by receiving eternal life, the gift of God which comes through Jesus Christ our Lord (Romans 6:23). The new life that we have in Christ right now is only the first instalment of what we will ultimately receive when we enter into the fullness of eternal life in God’s dominion.
This is aionios.