Just because something is ‘internally consistent’ does not mean it is ‘true’; it just means that it is ‘valid’. In other words, systems of thought that assume they are impervious to ‘critical’ engagement just because they can point to their logical coherence fail to recognize that ‘self-referentiality’ is not operating from a theory of truth that correlates to that assumed by scripture (‘correspondence’).
If the above is true then Islam is ‘true’, Mormanism is ‘true’, etc., etc. So what I am getting at, is that folks who believe that they have a sound system of thought (Calvinists) need to test what they believe (systematically and methodologically) from ‘outside’ of their system; they need to correlate what they believe (philosophical categories and all) with something or someone external to their system. If they don’t do this, they can certainly claim to be logically consistent within their own self-referencing system of conditions and premises; but not necessarily true (not ‘sound’).