It can be proved that, if the definition is dS=dQrev/T, the entropy is a state function for an ideal gas (a different definition would not ensure "statefunctionness"). That was the reason why Clausius chose dS=dQrev/T, but then he decided that the entropy so defined is a state function for ANY system. The proof he offered, however, was invalid. Since then the statement "entropy is a state function" has been repeated so many times that nowadays nobody would question it. Still it is just as unjustified as in the time of Clausius.

It can be proved that, if the definition is $\delta S=\delta Q_\text{rev}/T$, the entropy is a state function for an ideal gas (a different definition would not ensure "statefunctionness"). That was the reason why Clausius chose $\delta S=\delta Q_\text{rev}/T$, but then he decided that the entropy so defined is a state function for ANY system. The proof he offered, however, was invalid. Since then the statement "entropy is a state function" has been repeated so many times that nowadays nobody would question it. Still it is just as unjustified as in the time of Clausius.