If you have a question open a new post. Do not enter a dialogue chain opened by another for a different post months or years ago.
Legally speaking one need not belong to any religion. But if the party who gives you the application form insists you will have to go to court to assert your right. Further if the application is for membership of a religious body or if the constitution of the body insists that members should belong to a religion then the body can insist that you fill up the blank space left for religion also.
There is no straight answer to your question.