What arguments would you give to teenagers who are unconcerned about eternal life?

^{This is not really an answer so much as a perspective - that might, in part, apply. So I've marked it "wiki". If people think this is unhelpful, I'll delete it.}

As a youth, I was raised as roman catholic, and attended all the usual groups (including confirmation classes) and then some, including reading/serving in church, extra-curricular RE to GCSE (which was very uncommon, even in my RC school). I could have waxed on at substantial length for the equivalent of your catechism group.

But here's the thing: at no point did I ever believe one word of it. At best, I would have been (in my formative years) agnostic. I attended, and learned, due to expectation, and not wanting to disappoint. And, truth to tell, I enjoyed the learning. Religion remains of interest to me, but as an observer.

So: it could be that no matter what you say, you aren't going to reach (some of) them. As for what arguments I would give them... well, I wouldn't .

I don't know that the average teenager would find this answer persuasive, but here's how I see it:

Ask any teenager why he should go to school. Most understand that the purpose of school is to prepare you for adult life. You have to know enough math to be able to at least handle routine tasks like figuring out if you can afford that house or apartment on your income, etc. You have to know enough civics to function within our society's laws without getting arrested. You have to learn enough to be able to get a job when you graduate, like if you plan to become a chemist it would be a good idea to learn some chemistry, etc.

But when a kid is in school, he's not normally thinking, "I need to study chapter 4 in the math book because someday that will be useful when I'm trying to determine how much I need to save for retirement." Rather, he thinks, "I need to study chapter 4 so I can pass the test on Tuesday." The long term goals of preparing for adult life are often obscured beneath the short term goals of passing the test or completing the project, and are often ignored because of unrelated short term goals like winning the football game or getting a date with Susie. We know the ultimate goal is there, but we often forget it because of more immediate things.

It's the same way with eternity. The purpose of this life is to prepare us for eternity. But we -- teens and adults -- get wrapped up in the short term goals: getting a job, going on vacation, enjoying hobbies, etc. There's nothing wrong with many of these, and many are positive goods. But it's easy to let these immediate things become the focus of our attention.

There's a slogan that went around the business world a few years ago: Don't let the urgent become the enemy of the important. It's even more true about eternity.

I agree with David and Affable Geek's answers: teens tend to think they are indestructible so you might not break through. Lord willing, they will survive long enough to remember your wise teaching.

A suggestion that I have which probably can't be applied to your situation is to take a look at John Piper's Desiring God. According to Piper, God isn't just offering eternal life, but also a joyful life. The subtitle, Meditations of A Christian Hedonist, really grabbed hold of me. He quotes from C. S. Lewis' sermon "The Weight of Glory":

If there lurks in most modern minds the notion that to desire our own good and earnestly to hope for the enjoyment of it is a bad thing, I submit that this notion has crept in from Kant and the Stoics and is no part of the Christian faith. Indeed, if we consider the unblushing promises of reward and the staggering nature of the rewards promised in the Gospels, it would seem that Our Lord finds our desires not too strong, but too weak. We are half-hearted creatures, fooling about with drink and sex and ambition when infinite joy is offered us, like an ignorant child who wants to go on making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the sea. We are far too easily pleased.

Perhaps more useful is Blaise Pascal:

There once was in man a true happiness of which now remain to him only the mark and empty trace, which he in vain tries to fill from all his surroundings, seeking from things absent the help he does not obtain in things present. But these are all inadequate, because the infinite abyss can only be filled by an infinite and immutable object, that is to say, only by God Himself.—Pensees, 113

So what I would focus on is that heaven is really an extension of all the great things that this life has to offer, only better (perfected) and without end. What Christianity in this life is all about is preparing us to be able to enjoy the bounty of the next one. Being able to communicate that Christian truth in a way that really connects with them is a question I don't really have an answer for, however.

I'll let the Angelic Doctor have the last words:

I answer that, As stated above (Q[1], A[8]), the end is twofold: namely, the thing itself, which we desire to attain, and the use, namely, the attainment or possession of that thing. If, then, we speak of man's last end, it is impossible for man's last end to be the soul itself or something belonging to it. Because the soul, considered in itself, is as something existing in potentiality: for it becomes knowing actually, from being potentially knowing; and actually virtuous, from being potentially virtuous. Now since potentiality is for the sake of act as for its fulfilment, that which in itself is in potentiality cannot be the last end. Therefore the soul itself cannot be its own last end.

In like manner neither can anything belonging to it, whether power, habit, or act. For that good which is the last end, is the perfect good fulfilling the desire. Now man's appetite, otherwise the will, is for the universal good. And any good inherent to the soul is a participated good, and consequently a portioned good. Therefore none of them can be man's last end.

But if we speak of man's last end, as to the attainment or possession thereof, or as to any use whatever of the thing itself desired as an end, thus does something of man, in respect of his soul, belong to his last end: since man attains happiness through his soul. Therefore the thing itself which is desired as end, is that which constitutes happiness, and makes man happy; but the attainment of this thing is called happiness. Consequently we must say that happiness is something belonging to the soul; but that which constitutes happiness is something outside the soul.—Summa Theologica, Question 2, Article 7

Are they not into it because they do not feel it is real or because they don't care about the difference?

If they think that living forever (in some sense) and living only for a normal human lifespan are equivalent, you can invite thought-experiments about what things would be like for them in 40, 60, 80 years. Most teenagers have plenty of imagination to do that; they may not feel the finiteness as intensely as people who are older, but I would be surprised if they would agree that it wouldn't even make much difference to them.

If they think that living forever is not going to happen, either because they believe but don't think they can measure up, or because they don't really believe, then I would suggest that you are perhaps starting in the wrong place. For the I-can't-make-it crowd, you can emphasize forgiveness and Jesus' sacrifice for mankind (including them). For the I-don't-really-think-so crowd, you at least can probably get them to agree that there would be a difference if it was an option (back to the first point), and then leave it be until you're ready to talk about something that might be more compelling for them.

Unfortunately, some things that are professed in Christianity simply cannot be proven by any known scientific methods that I am aware of. It is, for the most part, a mystery to those of us who are still living. If the students are in a Catechism class, I would suggest simply teaching what is part of the catechism and referencing what the catechism says while also providing scriptural references.

An appreciation of the belief of eternal life that Christianity adheres to will likely come naturally in time, especially since even the young are prone to experiencing the passing of their relatives. However, there are some activities that can be taken to nurture that appreciation. For example, the corporal and spiritual works of mercy include such things as visiting the sick. Prayers for the faithful departed at a cemetery may also be something that can open people's eyes to the realities of the world and help people come face-to-face with their own mortality and grow in knowledge of themselves. Hopefully a better understanding of it comes through study and research. All a teacher can do is instruct about what it is that is believed, not necessarily change individual beliefs.

From my experience, I have found two things to help teenagers. :

1. When I talk about heaven, I'm always quick to point out that the "golden harp on a cloud" picture that we have of heaven isn't biblical, and frankly it would be kinda boring. Instead, I point out that eternal life with God is anything but static- we re still growing in heaven!

2. Don't focus as much on eternal life as much as real life in this present age. After all, we know that Jesus came in order that you might have life, and have it more abundantly. (in the here and now even!) Paul speaks of the fact that the joy of the Lord is our strength, and he admits that these things are written that " our joy may be made complete".

John Piper is a great resource for telling kids that the only way to truly enjoy life is to enjoy God. In enjoying Gods awesomeness, we learn what makes him so grat. (In fact, you might even say that God is most glorified, when we are most satisfied, in him). Teenagers already think they're immortal, so focus on the present until they realize their need. Once the experience the goodness of God, they'll realize they want to keep it forever.