One of the interesting questions about Christian moral philosophy is how moral life differs if Christianity is correct from how it would be if atheism is correct. Here is one difference. When I have culpably done wrong, I become guilty of the wrongdoing. The state of guilt is a bad state to be in. (It is good, however, if in addition to being guilty, I feel guilt.) If Christianity is right, then every state of guilt in this life has a potential cure through divine forgiveness. If atheism is right, however, then there will be incurable states of guilt.
There are two plausible ways of guilt being relieved. One of them is making sufficient restitution/satisfaction—as it were entirely undoing the badness of what one had done (I actually don't know if this really removes guilt—I think forgiveness may still be needed—but I don't need this for the argument). The other is accepting or maybe just receiving (it's a really interesting question which) forgiveness. But not just anyone can forgive a wrongdoing—the right person or persons must offer forgiveness. The most obvious thing to say here is that it is only those against whom the wrongdoing was done that can offer forgiveness.
If Christianity is right, every wrongdoing is also a wrongdoing against God. One can then argue that God has the authority to forgive the wrongdoing on behalf of all the aggrieved parties, say because all of the goods of all the aggrieved parties come from God, or because the aggrieved parties' very possibility of being better or worse off is a participation in God, or some such story. If this is true, then every wrongdoing can be forgiven by God, in a way that removes guilt. The defense of an exact account here needs more work, but it is clearly true that if Christianity is right, then forgiveness is possible.
But if atheism is right, then there will be wrongdoings which the wrongdoer cannot make sufficient restitution/satisfaction, whether due to the kind of wrongdoing (e.g., murder or rape), or due to the wrongdoer's lack of power (e.g., stealing money, then gambling it away, and then being unable ever to earn it back). Moreover, some wrongdoings of this sort will be such that it will be impossible to obtain forgiveness for them because the wrongdoings are against non-persons (e.g., wanton environmental damage, torture of non-human animals, etc.) or because for some other reason one or more of the victims are incapable of offering forgiveness (this will be the case if the victim is dead—by the wrongdoer's hand or not—or in a coma or the like). One might think that society as a whole can offer forgiveness on behalf of all victims. But that is implausible. First of all, a society plainly cannot offer forgiveness to someone whose crime was not against a member of that society. If Maxine wipes out an enemy tribe, forgiveness from a member of her own tribe will do nothing to remove her guilt. Likewise, society cannot offer forgiveness for the bulk of the wrong of torturing non-human animals (one might think society can forgive one for the parts of the wrong that consist of brutalizing society, or harming the animal's human friends or owners, but those are not the main wrong). Secondly, while one can argue that all wrongdoings are in a primary sense against God ("Against You, You alone, have I sinned," the Psalm has David praying) who is the first and final cause, and hence God's forgiveness suffices to remove guilt, many wrongdoings are clearly only secondarily wrongdoings against society.
This is not an argument against atheism or for Christianity. It is merely an observation of an important difference between the two. My feeling is that non-religious moral thought, however, mitigates the difference by not taking guilt to be as significant as Christianity takes it. But that mitigation is mistaken.