But in the conversation I was having, the criticism was raised, "Isn't heaven functioning here as a reward? And if so, hasn't heaven become transactional? Wouldn't the purest gift of love be one where there was no reward at all?"
This is a common criticism of heaven, that heaven functions as a "reward" for good deeds and, thus, undermines any true altruistic motivation believed to be at work in our sacrifices. We're always, in the end, looking for a pay off.
In response to this criticism, I'd say a few things.
First, the "heaven as a reward" criticism tends to frame the issue in the most childish register possible. Heaven is like getting a lollipop for being good at the dentist office. Or getting a sticker on your paper at school. I do Good Deeds so that I will get the reward of Heaven. This is a crude and simplistic, even intentionally derogatory, way of describing Christian motivation in light of heaven. The Christian worldview calls you to love, and it is love that motivates our actions. If I jump in front of a bullet for my family or face danger to protect innocent bystanders, I'm not making an economic calculation in my head: "I shall make this sacrifice so that I can go to heaven." I'm not looking for a trophy or a pat on the back.
I would argue that the issue of heaven is more about ontology than reward. If this physical life is all there is then an anxious ethic of self-preservation will, naturally, prevail. Sacrificial love is difficult to form in such a condition. But if Jesus was raised from the dead, if the tomb is empty and death has been defeated, certain moral capacities become more realizable. Not because I'm looking for a reward, but because there is more to life than just this life. And that "moreness" allows me to marginalize my animal instincts for self-preservation. Holding this life more lightly I'm more able to share it or give it wholly away.
Now, there is a retort here. Still, it will be argued, isn't it a much greater altruistic sacrifice if you give your life away for others knowing that there is no life after death? If so, only an atheist could be truly altruistic, as a lover with no hope of heaven.
My first response to this hypothetical is to question its realism. For example, has not my atheist interlocutor read Richard Dawkins' The Selfish Gene? If I act sacrificially to save my family are not my actions being motivated by my selfish genes who want to propagate themselves into the future? And if I rescue a drowning child who is not my biological offspring isn't my brain misfiring given that it has evolved in small kinship groups and just assumes any small child at hand has a high probability of being a biological relation?
Basically, if an atheist wants to get cynical about heaven I can get cynical about kin selection. Who can truly and transparently plumb the murky depths of human motivation? To say nothing of the fact that human motivation isn't simple and singular. We act in life with multiple motivations working at the same time. Most actions are a result from a mixture of both intrinsic and extrinsic motivations, each supporting and reinforcing each other rather than undermining or canceling. In fact, we often create and desire extrinsic motivations (like an accountability group) to steady ourselves when intrinsic motivations waver or fade.
Still, I get that the "altruistic atheist" is mainly being used as a hypothetical case to make a logical point about the nature of altruistic motivation purely defined. To which I'd say, I can think of many Christians who would also happily fill that space. If it was revealed to a Christian, right before a moment of sacrifice, that there was no heaven, would they still make the sacrifice? I can think of many Christians who would. And what would their motivation be? Love. In short, many Christians are happy to say that, even if there was no heaven, I wouldn't change a thing about how I have lived my life.
Which brings me back to the issue of ontology and moral formation. All of us, atheist and Christian, have lots of complicated and mixed motivations for the prosocial choices we make. Sussing those out and isolating them, in my opinion, doesn't get us very far. Nor is it very illuminating regarding choosing between a life of faith or atheism. Mostly because we're talking about hypotheticals with little relationship to the real world. So, for me, the issue goes back to what sort of world do you think you are living in and how does that vision of the world support or undermine moral formation? If you have a nihilistic view of the world, where acts of heroic love don't make a difference, you'll face some headwinds in forming love. In a nihilistic world, why should I undergo the hard pedagogy of love? Especially given that love is, in the words of Dostoevsky, a "harsh and dreadful thing"?
By contrast, if you possess a metaphysics of hope, where acts of love do matter, in an eternal sense, learning to love becomes a richer possibility. Our actions assume existential weight. This weight helps form love which will, when cultivated, begin to assume its own place and role in my motivational repertoire, and becoming, for the the saints, my sole motivation. As the Scripture says, three things remain. Faith, hope and love. But the greatest of these is love.
