Well, it is not a well defined term, but I will give it a try.
When I propose the nature of your character, based on my own interpretation of your belief, and criticize you on the basis of my own evaluation of that, it is bashing.
What you really believe is certainly not a part of that. In fact, what I really believe is not a part of that. I just took a cheap shot, because I wanted to belittle you, and your religion.
When I disagree with your religion, that might well be an impassioned and possibly acrimonious difference of opinion, or even difference of faith. But to postulate character elements on the basis of your faith, and ridicule you on that basis is not an argument to the opinion, it is a argument to the person. It is not logically germane, and has no merit as a counter argument to anything.
You can bash anyone. Atheists might be thoughtful people, unwilling to accept faith based beliefs, because of well founded beliefs of their own, not requiring faith. They might be dictionary thumping anti-religion proselytizers trying to feel superior to others. Arguing with them can be an intellectual exercise, conducted by logical methods, or it could be hysterical ranting meant to accomplish emotional goals unrelated to the existence of God. Assuming that they are stupid, evil, or even wrong because you believe differently is bashing.
An argument is not an attack. I can point out to you that other views exist, and explain why those views satisfy my need for understanding of truth. I can even listen to the reasons you find my views to be incomplete, or illogical. Argument is the exchange of new information. I already know that some people think Christians are weaklings, and that some find religion itself to be a delusion to escape from fear. Once you have let me know that you think that is the case, there is little other information you can give me. Reiteration of the same opinion isn’t argument. It is bashing.
“Reserve your right to think, for even to think wrongly is better than not to think at all.” ~ Hypatia of Alexandria ~