Actually, the doctrine of Communion of Saints, like all Catholic ideas, was an ever evolving one. Augustine held that Communion of Saints was nothing more than the body of believers, much like the Reformers. It was the ever encrouching false doctrine within the Church that changed the simple Apostle Creed to mean far more than was intended. The SemiPelagius St. Caesarius of Arles (c. 543), Faustus of Riez (c. 460), and others introduced their own errors into the doctrine such as praying to the dead. The Reformers simply took the meaning back to Augustine's view.
It goes back a tad farther than Augustine.