Eternity cannot be defined in terms of time, because time began to exist with this finite universe. Were we to define eternity in terms of time, we would need to give God a beginning, so that this entity we call God in reality would not be it; instead, it would be becoming more God as time goes by, as in the questionable “process theology.” Eternity must be something different. I say eternity is the plenitude that is experienced in a relationship of love. That is why God needs to be Triune in order to be eternal —the eternity of each of the three Persons in the Trinity would reside in the perfect relationship of love —given and received— they have with the other two.
Likewise, the eternity we long for, that which God placed in our hearts, must be of an intimate relationship of love with Him. In this way, nothing else, no one else, is able to satisfy such a longing, except the One who can love perfectly. Just Him.
But we are not there yet. However, in the meantime, the better our relationships of love here, the closer we will be to plenitude in this world. Maybe they are not going to be perfect, but it does not make them bad or less desirable. Everything that is finite is only small when compared to infinity. In this sense, no human relationship, no matter how loving, resembles having a relationship with God —the relationship with God there. Nonetheless, that which is finite can be large, very large, when compared to other finite things. Therefore, the more we come close to plenitude in our relationships of love here, the closer we will be to eternity, in a limit that is only going to converge eternally with Him there.
Meanwhile, we love here. We offer eternity —the eternity that God has set in our hearts— to those who are close to us here. And we do it not only with the hope that each of us is going to experience the eternity of His love there, but with the hope that our relationships of love here will be perfected there, and that, in Him, those relationships will also become eternal.