This is all similar to an objection to Kant's ethics of duty. Basically, Kant said that the truly moral act is the rational act - working out rationally what would be best if everyone did it, and doing that, whether or not you want to. That last part's important, because he also implied that it was more moral for someone to do their duty when they didn't want to than when they did want to.
So the problem is, good people want to do good things. But good actions are more moral when done by someone who doesn't want to do them.
Anyway, taking Chris Christian and Andy Atheist in the OP, they both give to charities and stuff, and Chris believes he'll get rewarded in Heaven and Andy doesn't, so does that make Andy's actions more moral than Chris'?
Firstly, what are we assuming about what makes an action moral? Obviously, we're assuming that an action done for reward is less moral than an action done for no reward. We're also assuming that Andy's action is done less for reward than Chris'. But is that really true? Doesn't Andy get something out of it? He gets the pleasure buzz of having helped someone, and perhaps he avoids the unpleasantness of guilt that he would have if he passed up an opportunity to help someone.
Secondly, we're assuming that Chris believes he'll be compensated for his generosity in Heaven. I don't think many Christians really believe this. For the most part, Christians believe that they can't possibly do anything to deserve any kind of compensation in Heaven. This doesn't necessarily help, since their actions may be motivated by feelings of guilt, and that doesn't smack of morality much to me, either. Ideally, the Christians should be doing good with no thought to himself or his actions at all. Ideally, the charity of the Christian comes naturally, because the Christian has worked to reduce his own selfishness in order that God may act through him.
The whole thing comes down to what counts as a "good" motivation to do something moral. We don't do things unless we have a reason to, and the OP implies that having a reason to do something moral makes the deed less moral. The closest we can come is when we ask someone, "Why did you do that?" and they say, "It was the right thing to do." But then we can still ask, "Why do you do the right thing?" and there's a good chance that, with enough honesty and interrogation, we'll learn that Joe Interrogated would feel bad if he didn't do "the right thing" and feels good when he does "the right thing".
But then, I'm an atheist, and I don't believe in any objective morality, so the whole thing's moot to me.
