I would disagree. In a theocracy, the concepts of law and religion are one and the same.

Thats an extremely narrow view of what a theocracy, Iran by your description does not qualify as a Theocracy (In fact no government in the world qualifies by this standard)

In modern Western societies (although the Ancient Greeks also based their socio-political order on this principle) the law is built more on the system of philosophical thought.

Well your knowledge of Greek government is sadly lacking too (In fact a majority of  ancient Greek city states would qualify more as Theocracies than Iran would) The idea that Modern western government is based on systems of philosophical thought is stretching things extremely. Influenced by yes, but ascribing a secular basis is an amazing reach that should be beyond any honest consideration

The latter can change basd on social priorities at a given time. The former is immutable.

Well that statement avoids any knowledge of history The world changes, so even the most severe Theocracy is going to be faced with changing situations, making Immutability an impossibility.  Theocracies also have  a history of changing as social priorities change, for example,  the marriage of priests in  the Vatican has at times been allowed and not allowed, and has changed in accordance with social changes  inside the church

So Basically on the three major points inside your statement, youre wrong on all of them, badly

Any idiot can face a crisis - it's day to day living that wears you out.