Fundamental duties can't be enforced just like the directive principles, they are present to "guide" people. But, what can be enforced is section 295(A) of IPC, I am pasting it verbatim here.

1[295A. Deliberate and malicious acts, intended to outrage religious feelings or any class by insulting its religion or religious beliefs.— Whoever, with deliberate and malicious intention of outraging the religious feelings of any class of 2[citizens of India], 3[by words, either spoken or written, or by signs or by visible representations or otherwise], insults or attempts to insult the religion or the religious beliefs of that class, shall be punished with imprisonment of either description for a term which may extend to 4[three years], or with fine, or with both.]

So what would happen if you criticize religion or beliefs is pretty obvious here. Indian Constitution has pretty generic riders for freedom of speech, the impetus is on the executive mostly and with the current state of the executive we are in for a rough ride.