An absence of religion imparts more robustness to a constitution than an amalgamation. It leads to too many opposing opinions, most significantly on aspects that fundamentally differ among religions as they are practised and/or interpreted, as well as between religions and modern social culture. Every religion as practised possesses backdated, illogical and barbaric ideas. As a civilization progresses it is important to identify those malignant points and get rid of them. Unfortunately, there are a vast majority of practitioners or, as it is often put, "theists" who believe in the immutability of instructions, so much so that many of them rather indulge in practising a form of the religion that can even be as much as a few millennia old. Given the social impact of religions, this is rather devastating. It voids the progress human knowledge has made over this period of time. Failing to encompass the evolution of a mass conscience that has been achieved through rational reasoning and practical experience, is a serious folly of any set of social guidelines. Denial of truth is self-defeating.
The Indian constitution, through its penchant for 'equality', theoretically empowers such out-dated opinions with equal significance as the most modern sociological ones.