Unfortunately for these folks, though, they're missing the point. The issue is not whether people can be kind, generous, compassionate, faithful, etc. if there is no God, of course they can.
Rather the issue is whether there can be any meaningful moral standards if there is no God. If there's no standard of morality outside ourselves and no accountability for how we measure up to that standard, morality is just a bundle of personal preferences and no one's preferences are any more "morally right" than anyone else's.
C.S. Lewis had something to say about the man who asks why we should think he can't be good without believing in Christianity. Lewis states that the man is not asking a reasonable question: "If [the man] never heard of Christianity, he wouldn't be asking this question," and "if, having heard of Christianity, and having seriously considered it, he had decided it was untrue, then once more he would not be asking the question."
Lewis says that the man who asks why he cannot be good without believing in Christianity,
has heard of Christianity and is by no means certain that it may not be true...to such a man it might be enough to reply that he is really asking to be allowed to get on with being "good" before he has done his best to discover what good means....He is deliberately trying not to know whether Christianity is true or false, because he foresees endless trouble if it turns out to be true....This man is not committing an honest error, Lewis asserts. Rather he's committing a dishonest error. Lewis doesn't put it this way, but the man is deliberately refusing to examine his reasoning. He's refusing to consider how there can be moral right and wrong if he, or society, is the highest moral authority. He's refusing to consider how anything can be morally wrong if there's no ultimate accountability for how people live. He's refusing to ask himself what the words "morally wrong" even mean if we're just the product of mindless forces and random chance.
He is like the man who won't go to the doctor when he feels a mysterious pain because he's afraid of what the doctor may tell him....
If the atheist is correct, if there is no God, no transcendent moral standard to serve as an objective reference point, then there's absolutely no reason why people should care about those they have no feelings for. There's no reason why anyone should sacrifice his own well-being for the sake of others. There's no reason why we should not all be egoists or even nihilists.
The person who thinks that there are things that are objectively wrong, who thinks that it would be morally wrong to be an egoist or a nihist, is tacitly acknowledging that there must be a God.
The only reason human beings have for not being egoists or nihilists is that they believe that it would be objectively wrong to be such a person, but it can only be objectively wrong to be such a person if there's an objective standard of moral right and wrong. And there can only be an objective standard of moral right and wrong if there's a God.
There's nothing else that could bear the enormous weight of human morality.