To whom are you reading your psalms, David?

[Ducklet Cat]

I've run into the expression:

To whom are you reading your psalms, David?


And we do use the same sentence in Arabic to mean: Whom are you tricking?

Does the expression mean the same in English?
Or does it mean: "What you are doing is in vain"?


Thanks.

[5jj]

I have never come across this.