(Got a question from one who heard that “eternal” means a very long time. Therefore, the “eternal life” offered by Jesus may only be a very long life which could end at some point in the future. This is the brief response I wrote)
God does not offer “eternal life” as a shadow or a trick or some temporary thing. God holds eternal life up as one thing so valuable that it is worth losing our life to gain this eternal life. It is better to be hated, abused and murdered and gain this eternal life, than it is to have every good thing which could be had in this world.
The fact that God offers it to us, should give us comfort. If God offered a life which might run out, then it would disturb our peace:
It is an endless and everlasting life. Such as are once possessed of it shall never be dispossessed again. If man be designed to enjoy a chief good, and this chief good must content all our desires, it must also be so firm and absolutely immutable as to secure us against all our fears; for a fear of losing would disquiet our minds, and so hinder our blessedness.
Thomas Manton, The Complete Works of Thomas Manton, vol. 11 (London: James Nisbet & Co., 1873), 366. God has not offered a very long life as our supreme good. God has offered us a life which is both never-ending, but also which belongs to a differ age, the age to come. Both of those things should give us comfort.
First, when we speak of “eternity” and God, we must out of our heads the idea that “eternity” is a very, very long time. This is hard for us to do, because we only have only experienced time in this way. In Romans 8:20, Paul explains that the creation – the entire universe that we could know – “was subjected to futility”, it is vain, it is running down (Eccl. 1:2, Gen. 3:19).
This matches what we know about the universe from observing it. Physicists talk about “Time’s Arrow”: the universe is running in one direction, and it is running down (https://en.wikipedia.org/wiki/Arrow_of_time). So when we talk about “time”, we think of a succession of moments and an increase in entropy.
Stephen Charnock in The Existence and Attributes of God writes
We must conceive of eternity contrary to the notion of time; as the nature of time consists in succession of parts, so the nature of eternity in an infinite immutable duration. Eternity and time differ as the sea and the rivers; the sea never changes place, and is always one water; but the rivers glide along, and are swallowed up in the sea; so it is time by eternity.
There is a great deal of discussion and speculation when it comes to what eternity actually means. Eternity – and infinity — are very strange and very hard concepts. God is called the “everlasting” or “eternal” God (Rom. 16:26), he is the eternal king (1 Tim. 1:17). That is why in Revelation we read that God was, is and is to come (Rev. 1:8, 11:17).
When we start to think of concepts like “eternal life” (John 3:16), we have to realize that when it comes to divine things, we are not speaking about very long things.
It is true that sometimes the words translated “eternal” or “everlasting” sometimes have the idea of very long, or indefinite, or “age”, or “aeon”. That, however, should not trouble us. When we speak to one-another we often talk about something “taking forever”, when we mean 20 minutes. We will say that it was “an eternity”.
But we can also use the word “forever” and understand it to mean something which cannot end. When we use the word “forever” or the word “eternity” we can tell what we mean – and we expect other people to be able to understand us easily. We do this, because can understand the context and the use. We understand that sometimes a word is being used ironically, or emphatically. So if I tell my wife, I will love you forever, I mean to underscore the intensity of my commitment: even though we both know that neither of us will literally live forever.
The same thing applies to uses in the Scripture – the Bible is written in ordinary language. So in Genesis 9:16, God makes “an everlasting covenant” to never flood the earth again. But we also know that God will one day re-create the entire universe (2 Pet. 3:7). Therefore, we know that this covenant to never flood the earth will hold true throughout the duration of the earth’s existence, but the covenant does not mean that God will keep the earth in existence forever.
Or in Genesis 17:8, God promises Canaan as an “everlasting possession” – we quickly see the problem of simply using the word without consideration (even if we decided we would think about it forever).
So, in some places the word aion/aionios means a long time ago: Luke 1:70, As He spoke by the mouth of His holy prophets from of old (aion).
In context we can tell it cannot mean “forever” – that would result in nonsense.
By contrast, in 2 Corinthian 9:9, we read that God’s righteousness endures forever. We can’t say that God’s righteous will last a long time and then wear out. Or God’s throne is “forever”. (Heb. 1:8). If God’s throne is not going to last, God is not much of a God.
What I want you to see here is that you cannot fear that our promised eternal life will wear out in the distant future merely because the word “aion” could mean a very long time. Our word “forever” can mean “a long time”. The way in which a word could be used does not tell me how it is being used.
Second, when it comes to eternity and God, our normal concepts of time simply do not apply.
How then is the word “eternity” used when it comes to our “eternal life”?
It would make very little sense to say that you will live “forever” and it to be only a very long time. Life is something which one either has or does not. If life is everlasting, the word “everlasting” or “eternal” would not be ironic/hyperbole (“it took forever to get home”).
It could be emphatic: and there is a sense in which it is. It does not merely mean continual and without end: it means life which belongs to another age: thus the language life of the Age, or Aeon would point toward not merely a long life, but a life which belongs to the age to come, to “eternity”.
But perhaps the most important aspect is that the idea of “eternal” life is contrasted with death. Consider John 6:51 & 58. In this passage, Jesus is contrasting the bread eaten in the wilderness (manna) which himself as the bread of life. Jesus notes that the fathers ate manna and died (“Your fathers ate the manna in the wilderness and died” John 6:49). Yet the one who eats Christ “will live forever” (John 6:50). He repeats the same idea in John 6:58: they ate and died, but “he who east his bread [Christ] will live forever”. If Jesus is merely offering an extremely long life, this argument fails. Jesus’ offer is something that cannot end, or his argument is a lie.
This argument is stronger when you consider the other concepts and images which are used to complement the idea of “eternal life” in John 3:
That is the immediate result of the love of God for the world: the mission of the Son. His ultimate purpose is the salvation of those in the world who believe in him (eis auton, not en autō as in v. 15). Whoever believes in him experiences new birth (3:3, 5), has eternal life (3:15, 16), is saved (3:17); the alternative is to perish (cf. also 10:28), to lose one’s life (12:25), to be doomed to destruction (17:12, cognate with ‘to perish’). There is no third option.
A. Carson, The Gospel according to John, The Pillar New Testament Commentary (Leicester, England; Grand Rapids, MI: Inter-Varsity Press; W.B. Eerdmans, 1991), 206. Eternal life runs parallel to born again. We cannot be “unborn”, therefore, by analogy we do not un-live.
Second, the contrast is made to death and destruction. If we will die, then the offer of “eternal life” makes no sense if “eternal” only means very long time.