When you think about what this is really easy to see is a = b. That I am rewarded with promise of heaven for a deed, or hope of being well-respected by others who see what I've done, affects the same object: ego. It's not driven by desire to help others (compassion, love or else), but desire to be seen /judged as good by others.
There is a c) (and more for sure). The much more mundane reason c) is "give to people and let your reward be the feeling you get that you do a good thing". Good thing can be in this case simply the result of mental congruence of your act with, say, your utilitarian belief that "helping others by decreasing their suffering is good".
If I tell people "a good act is when you help others to help them maximize utility", it's a moral stance. It's a very different moral stance than "a good act is when you do what someone says you should do in order to get eternal life".
The former is far better from ego-point of view, and less prone to corruption, and focuses on the values and reasons why somethign is good, in other words - promotes understanding and is clear why it's good. The latter promotes blind following and subservience to anothe person/being becomgin the guide of y our actions.
It's clear that most people need a guide, but be careful what kind of guide you pick.
Consider also the story of the Good Samaritan: Jesus tells the story of a person who goes out of his way to help someone in need. He incurs risk (helping the man meant risking getting caught in an ambush), expense (oil and wine for the wounds plus paying the innkeeper), and inconvenience (he had to interrupt his journey to help). Jesus tells us this is how we should treat our neighbor.
The Samaritan is also assuredly not Christian so isn’t doing this to gain eternal life in heaven through faith in Jesus. It’s pretty obvious and indisputable altruism.
Instead of giving publicly to receive an extrinsic reward, instead you should give in secret to receive a different but still extrinsic reward.
Of course, a non-believer donating in secret is also probably doing it in order to obtain an intrinsic reward of self-satisfaction, and there’s no real reason why I should view the intrinsic reward self-satisfaction as any more worthy than the extrinsic motivation of God’s reward.
I don’t have any really coherent point here, it just seemed somewhat interesting…