"because I am not being rewarded during my time on earth, that is why it is a selfless act."
I know that, it was in my question. So I'll rephrase it: WHY is the fact that it happens after your time on earth relevant in any way regarding selfishness and selflessness?
It's still you, your motivation for your actions was: your reward after your death, which makes it selfish, just as a person having his motivation for his actions being a reward during his life.
"I don't gain anything by putting others before myself, especially when it is inconvenient. "
Well of course you gain something, that's why you said that "God rewards you in heaven". That's your gain right there. After all, would there be a reason for you to act "selflessly" if your divinity wasn't rewarding you after your death?
"Yes, someday I will be rewarded but that is because I am being rewarded for my faith and willingness to be selfless."
And yet the fact that it's what motivates you to act "selflessly" makes it selfish in some way as well, since if you didn't have this desire for being rewarded after your death, you wouldn't act "selflessly". Or would you?
"You can think about it like this, an act of selflessness does not guarantee a reward from God unless it is truly a selfless act. God knows our hearts so he will know if I am really doing it to score points (for personal gain), or if I'm doing it to really help another person. That's the difference."
I'll set aside the fact that you haven't established the existence of your divinity for now. How would YOU know whether or not something is truly a selfless act? Seems important if you're going to do these actions. Can only your divinity know or can you know as well?
"If someone does a selfless act on earth and doesn't believe in God, good for them, but you cannot argue that what they did was morally good without assuming a Christian worldview. A secular view cannot argue that this is morally good."
I know it's frequent for Christians to assume that they hold the monopoly of being morally justified, but it surprises me every time I see the claim come up.
Of course secular views can argue for what's morally good, there are many different views on morality and many of those (but certainly not all) are secular. There are some views which argue that pleasure is the source of morality, others that argue that well-being is the source of morality, others that argue that desires are the source of morality, etc...
Of course, just like Atheists or believers other than Christians reject the idea that your divinity is the source of morality, you're free to reject these views as well, but please stop claiming that Christianity is the only view to have a basis for morality, that's not only false, but also nonsense.
"A popular secular humanist point of view is that morals come from survival of the fittest, meaning that nothing is good unless it is done for the purpose of surviving. This sounds logically coherent and you can apply it to many aspects of your daily life. We eat healthily, we exercise, we treat others kindly because they can help us survive."
That seems quite limited, I doubt it's a humanist point of view (secular or not) . Humanists don't only care about survival, they care about what most humans care about, which is living good lives, for as many people as possible, so that's not limited to mere survival, but includes having decent means of living (not just surviving), having rich and fulfilling goals, being surrounded by interesting and cooperative peers, etc...
But even if it was true that there was someone (let's call him Benny) who would argue that survival is the source of morality. Then Benny would have his own view on morality, and without requiring Christianity's. Whether or not you would agree that his view is true is another matter, whether his view would ACTUALLY be the correct one is also another matter, but regardless, he would have one without requiring yours.
"All of this seems like it makes sense until you realize that there are people out there that sacrifice themselves daily for others for no gain. These are selfless acts, you can see them from just the tiniest act of service to the extreme and rare act of dying for someone. Romans 5:6-8 explains how Jesus was the perfect example of a sacrifice and doing a selfless act."
I agree, some people sometimes sacrifice stuff for others, sometimes their own life. So? Those are things that are explained by most models of morality out there (secular or not).
Benny, our "survival-of-the-fittestist" would tell you that a mother would sacrifice herself for her child so that her offspring would survive and carry on her genes as well as the species. That would make it moral in his model.
A strict utilitarian would tell you that he/she would die for 5 strangers because it ultimately maximizes well-being and minimizes suffering as a whole. That would make it moral in his/her model.
Some believer could sacrifice himself/herself for someone else because he/she believes his/her divinity approves of it. That would make it moral in his/her model.
" Elaborate on this if you don't mind; how can it be unhealthy?"
Well I thought I explained how in my previous comment: it can be unhealthy because by sacrificing too much for others, you could hurt yourself and/or your ability to contribute to society in the future. Worse, you could even hurt yourself and be as a result unable to help the person you intended to help in the first place.
For example: if you and your friend are being mugged, assuming you know self-defense and he/she doesn't, you would be wise to defend yourself first if you want to be able to defend him/her. Or if you're in a plane which is having a problem, it's vital that you put on your mask first, and only then start helping others. A medic on the battlefield will need to make sure he's alive (and able) if he wants to be able to save his comrades.
Those are examples of situations where being selfless is not advantageous, neither to you nor to other people, and could even end up with both you and the person you want to help dead.
But even on the long term and more daily scenarios, sacrificing too much into helping others means that you're neglecting your own desires, which means that you're exposing yourself to depression or other mental struggles, which means that you're less able (if at all) to help others in the end.