Historically? The Cathars emerged in the 12th century, and don't get any ruler-level characters until much later (I believe Raymond VI of Toulouse is classed as a Cathar by the early 1200s, but I'd have to check).
You'd be better off as a Bogomil; they're Orthodox heretics, fairly similar, and originated in the 10th century.
Both strands come out of Gnosticism, which is a mystic, dualistic movement that has been running around in some form or other since early in the Church's history, so if you really want, you could justify it as being not-really our Catharism, but another gnostic movement with the same name (it literally means "the Pure Ones", which is generic enough to apply to anyone who wants, and indeed there had been at least one other, completely unrelated heresy that had used the same name in the 300s).