By a result of Kadison, every operator subsystem of a commutative $C^*$-algebra is isomorphic to the space of continuous affine functions on its state space.
In other words, if $X$ is a compact Hausdorff space and $E\subseteq C(X)$ is an operator system, then $Aff(S(E))$ is isomorphic to $E$ (as operator systems- complete order isomorphism), where $S(E)$ denotes the state space.
This seems to be a generalisation of Gelfand Duality for commutative $C^*$-algebras.
I would be happy to get a recommended reference for a proof.
Moreover, I could not see why in the case that $E$ is a unital, commutative $C^*$-algebra, i.e. $E=C(X)$, then we actually get $A(S(C(X)))=C(X)$ as expected? Since $S(C(X))$ can be identified with the space of Radon probability measures on $X$, $P(X)$, my question reduces to the question: Why the continuous affine functions on $P(X)$ are isomorphic to $C(X)$?