We usually conceive eternal life as 'going on forever' into the future. Many have pointed out that, if that is true, eternity starts now - rather than when we die, as is often intimated. Others have gone further to indicate that our English translations of words like 'eternal' fail to capture the original Greek and Hebrew meanings, which relate not so much to duration, but rather to quality. In this post, I wish to make a minor addition: eternal life relates not only to the future and the present, but also to the past. Not only is eternal life never ending; it is also never beginning - that is, always existing.
Due to the frequency and ubiquity with which olam occurs, it would basically require a complete study of the Old Testament to elucidate its full significance. Hopefully, a single example will suffice to demonstrate the point of interest. In Jeremiah 6:16, the Lord tells Israel to "ask for the ancient paths". David appears to obey this commend, asking the Lord to "lead [him] in the way everlasting" (Psalm 139:24). The same word - olam - is translated as "ancient" in the first case and "everlasting" in the second. One rendering involves the past; the other, though less definite, seems to involve the future.
It is noteworthy that olam is mainly used to describe the the sacred facets of God - His existence, His covenant, His laws, His promises, His word, and His kingdom. These facets remind us that God is outside of time; and when this is taken into account, the dual meanings of past and future eternity can be construed as merely two sides of the same coin. This is clarified by the phrase "from everlasting to everlasting" - "from olam to olam" - which appears in three Psalms: once to declare praise to God (41:13), once to describe the nature of His existence (90:2), and once to characterise His lovingkindness (103:17). God is "the Alpha and the Omega, the first and the last, the beginning and the end" (Revelation 22:13, emphasis added); He is the Great I AM (Exodus 3:14).
An analogous word in the Greek is aiónios (from the root aión), which is of particular interest due to its appearance in the verse most commonly associated with 'eternal life', namely John 3:16. Like olam, aiónios denotes a state of being that is simultaneously 'without end' and 'without beginning'; indeed, it is perhaps more explicitly indicative of timelessness (and quality) rather than merely perpetuity. Like olam, it refers to the things of God - His redemption, salvation, and glory, and also His judgement, punishment, and destruction. Moreover, in parallel to the Old Testament phrase "olam to olam", in Revelation 22:5 John envisions that the servants of God will reign "eis" - that is, throughout, beyond, or even before - "the aiōnas of the aiōnas".
How is it possible for us to assume the infinitude of God? The author of Hebrews (9:14) gives us the answer when he speaks of "the Spirit aiōniou", i.e. the eternal Spirit. That same Spirit has been joined with our spirit (1 Corinthians 6:17), which acts as a guarantee of our coming glory (Ephesians 1:13; 2 Corinthians 1:22; Colossians 1:27), testifying that we are children of God (Romans 8:16). The more Spirit and the less flesh that we become - and therefore the more like God we become (2 Corinthians 3:18, 5:17; Ephesians 4:22-24; 2 Peter 1:4) - the more aiónios and olam we become. We become not just people who will always exist, but also people who have, in a sense, always existed. Our past and future status in turn sheds light on our present - which, indeed, is often the tense in which both the Hebrew and Greek words are employed.