We seem to have stumbled into the error of thinking that we discover what eternal life is by looking forward. Most of us are aware that longevity says nothing about its quality and have embraced the words of Jesus that eternal life is “that they know you, the only true God, and Jesus Christ, whom you have sent” (John 17:3). But despite knowing this, we still persist in speaking of it in terms of time, and for most of us that means a construct of linear time stretched out to infinity. We continue to have a predisposition to look forward.
However, it would seem logical that if this life is eternal then we should not only look forward but also look back, and ask “What was (is?) the nature of life that existed in eternity?”
And as those who have embraced the mystery of Trinitarian theology we know that from all eternity the life of the godhead is one of giving itself away (otherwise God could be loving but he would not be love). The Father gives himself away exhaustively in the generation of the Son, and the Son reciprocates in pouring his love back upon the father. The Spirit then becomes, in the words of Augustine, “the bond of love between them”.
So if we want to understand eternal life it would seem obvious that the more we can understand about the life that existed in God from all eternity, then the closer we might come in understanding how we can embrace that life both now, and in the future. And if that life consisted in the persons of the Trinity giving themselves away to each other eternally (and at some point, giving themselves away in creation) then we have a template for understanding how the Sprit is striving to birth the same life and love in us.
Then, if we take this one step further, we realize that eternal life is therefore a missional concept. It is all about giving ourselves away (which is painfully obvious, even if we only had the great commandment and the great commission to go on!).
And I think all this is beautifully expressed in the words of 1 John 1:2 where we read “and we proclaim to you the eternal life, which was with the Father and has appeared [gone forth] to us”.
So, to sum up, we begin by affirming that eternal life is something qualitative, not just quantitative (John 17:3). Then instead of defining it by just looking forward, we change direction and start looking back. There we discover that the life that existed eternally in God was a life that has been eternally giving itself away (Trinitarian life), and that creation (and the cross) was simply the overflow of this self-giving love. And finally, that eternal life must then, of necessity, be a missional construct.
Much more could be said, but for me, this is not a bad place to begin.