What is the ruling concerning a Muslim who describes another Muslim as "godless" and justifies his intention by saying that he is "immoral"? And what is the ruling concerning someone who is asked about this person and states that he is a Muslim?

It is not permissible to describe a Muslim as godless (without religion), because it is a grave accusation of disbelief (kufr). Moreover, backbiting is forbidden, and it is mentioning your brother with what he dislikes. If what you say is true about him, then you have backbitten him. If it is not true, then you have slandered him. And upon him is to repent to Allah.