sit in the middle of the bell curve, etc.

The bell curve is a mathematical (i.e. statistical) concept that roughly says that in many things, most people are average (only a few people are outside the norm). You can read about it here. https://en.wikipedia.org/wiki/Normal_distribution If you look at the graphs in the first diagram, they look kind of like bells (hence the name).



To "sit in the middle of the bell curve" is a overly wordy way to say "to be average".

Yes, BS is short for "bullshit".

The term "god-fearing" has the implication of someone who follows God's laws religiously (pun intended). If you fear God, then you do whatever God says. This means that you are going to avoid sin.

Thank you very much for your help.

Is "sit in the middle of the bell curve" an idiomatic expression? If yes, I want to write down in my notebook of English.

No, it's just something native speakers would understand even if they never heard it used before.

Thank you for your help.

I have never heard this expression before. It actually sounds a little awkward to me (I wouldn't recommend you write this expression in your notebook). As Roger says, most Americans know what a bell curve is. So when I read this, I knew exactly what the writer was talking about.

It is important to know what a bell curve is.

Thank you again.