Eternal Life Now

Though [John's gospel] affirms life after death, his phrase "eternal life" does not mean primarily that. The English phrase translates to a Greek phrase that in turn expresses a Jewish notion: "the life of the age to come." "Eternal life" means "the life of the age to come." Thus, for example, John 3:16 could be translated

For God so loved the world that God gave God's only begotten Son; whoever believes in him shall not perish, but shall have the life of the age to come.

[Importantly] in John, "eternal life" is often spoken of in the present tense. "The life of the age to come" has come. It is here. Eternal life does not refer to the unending time beyond death, but to something that can be known now. "This is eternal life," John affirms, and then adds, "to Know God." To know God in the present is to experience the life of the age to come. It is a present reality for John, even as it also involves a future destiny. We can know it now, experience it now. The point is that even John's language about "eternal life" has a strong present dimension.

-Marcus Borg, The Heart of Christianity, pg. 175