Oddly, even though I am an atheist, and I know the word is religious in origin, I use the word sin from time to time, and “believe” that some things are a sin. The way I use it however, is just as a description. I don’t believe in any god that could be there declaring what is and what is not sin, but I will consider some things to be a sin.
Really, the way I use it, is just as a scale of how bad something is. I would only use it to emphasize just how bad something is.
I will also use words like soul and gospel, in a non musical sense, also without the religious belief part of the word.
The words soul, sin, etc. are useful to some extent, so I will use them. For example, I would have no problem saying “that girls ass stirs up something in my soul” or “wasting a good steak is a sin”.
In regards to believing in sin, then I also kind of have to say that I do. I do really think that it is a sin and/or sinful to do certain things. Mostly, I will use it in relation to waste or wasting things, and in relation to comedic exaggeration.
The most traditional use I give the word, is in relation to the topic of greed. I would not have any complaints if someone used the word sin to describe some of the things that banks get up to.