As a theologian, I consider that “religion” and ‘religious” are not just terms related to a church, mosque or synagogue. During Plotinus’ lifetime the various cults of deities — the religions of antiquity — was still in full panoply, and some had not disappeared by Augustine’s time. I think you are referring to his debate with Pelagius, who certainly did not think salvation was by one’s own merits alone.

In any event, there are religions that have no clergy or rituals, even along Christians: the Society of Friends, for instance (the “Quakers”).“Religious” refers to a person’s horizon concerning ultimate things — birth, death, and identity, among other questions. It consists of the beliefs that “tie together” (re-ligio) to form that horizon, and that horizon usually changes with time.