Atheists have no expectation of reward when doing something for others so it's my opinion that only atheists can truly be selfless in their acts.
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Well, that atheists (you speak of / for all atheists?) have no expectation of reward is an assertion that warrants evidence if it is to be trusted. I see no evidence that, on a wholesale basis, your assertion holds any water. I would, however, agree that someone who is atheist would likely not expect a reward after death. Rewards come in many forms. Sometimes, it's not actually a reward one might be after but, instead, indebtedness (which can come in quite handy). So...I disagree with your OP.
Altruism by definition is selfless. If someone is expecting a reward of any kind then their action is not altruistic. Making someone indebted to you is not selfless. When someone is indebted it is expected that they satisfy said debt in some manner, i.e. some reward. Whether it be eternal bliss, cash or a pat on the head, it's still a reward.
I don't think there is such a thing as a "selfless act" in the strictest meaning of the word, in that we all get something out of everything we do. When I do a good thing for another person, it makes me feel good,...that is my reward. I honestly think that is the motivation for most religious people as well. I think that it is quite arrogant to think that because of our lack of belief in deities, we as atheists are somehow better people than religious folks. The vast majority of human behavior is fueled by reward, or at least the avoidance of distress. It matters not if you believe in a god. That being said the old saying of "good people do good things, bad people do bad things but to get good people to do bad things you need religion" still fully applies.
"When I do a good thing for another person, it makes me feel good,...that is my reward."
In which case it's not an altruistic act.
"I think that it is quite arrogant to think that because of our lack of belief in deities, we as atheists are somehow better people than religious folks."
There was no better or worse involved in the topic, only different.
Regardless of religion or lack thereof, if there is an expectation of reward then it's not altruism.
If ones personal philosophy is that by being good they avoid eternal damnation and are rewarded with eternal bliss then no act they perform can be altruistic as there is always the underlying expectation of reward.
On the other hand, as atheists don't anticipate eternal bliss or eternal damnation they can actually do something without expectation of reward of any kind.
As an example: I pulled a man out of a burning building. If I were religious I would expect that I scored points towards heaven with my god. Since I'm not religious I didn't and don't expect any reward. It was an altruistic act.
I tried being altruistic once. It didn't work - I kept on getting rewards back 10 fold. Now I'm just an asshole :P