The Achilles Heel of All Religions

Almost by definition, all religions make unfalsifiable claims.   This presents two problems for religion:  

First, by definition, there is simply no way of vetting or validating any unfalsifiable claim or of qualitatively differentiating one from another.  This means that, as a matter of pure logic, all unfalsifiable claims are equally (in)valid.  

Second, because unfalsifiable claims are unconstrained by any requirement that they can be falsifiable, the potential number of such claims is literally limited only by imagination.  Consequently, we see an explosion of contradictory claims among, and even within, various religions.  

Put one and two above together and you come to an inescapable conclusion:  There's no principled way to choosing between a nearly endless number of unfalsifiable religious claims, or of persuading others to your way of thinking.  Any decision to accept any particular unfalsifiable religious claim is therefore one made purely from blind faith. 

Additionally, lacking the aid of principles, any effort to evangelize or convert others to your favored claims quickly deteriorates into string of logical fallacies--ad hominem, non sequitur, cherry picking, straw man, etc.  

When people are forced to argue their stance using fallacies rather than logic or reason, frustration, agitation, and anger reign.  All too often these emotions reach a boiling point.  Social disharmony, and too often actual violence, result.

Consequently, despite that most religions preach peace, the history of religion is very much a history of unresolved debate, conflict and violence.  Until the 20th century, when the influence of religion began to diminish, many of history's greatest conflicts were anchored in religion.