Just found this interesting tidbit of information. I always thought the Oneness doctrine was fairly new – late 19th century at least. But then I read about Sabellius.
The New World Encyclopedia says:
Sabellius, a Christian priest, theologian, and teacher, was active during the first decades of the third century, propounding a Christological doctrine that was later deemed heretical. Specifically, he advocated a modalistic view of divinity that described God possessing a single unified substance, albeit one that took particular forms (Father, Son, Spirit) in relation to human beings. As this doctrine denied full, discrete reality of each “Person” of the Trinity, it was anathematized, leading to Sabellius’ excommunication from the church in 220 C.E.