Sabellianism believes in the full divinity of the Father, Son, & Holy Spirit, but denies they are eternally distinct persons.
Anyway we blogged about the story of Belknap on Watts here. A taste from the mouth of Rev. Belknap:
"With respect to the idea of Personality, as applicable to the Father, Son and Spirit. Dr. Watts differed from many Trinitarians, as he denied (and I think with sufficient reason) that there are in Deity three distinct Infinite Spirits, or really distinct persons, in the common sense of that term, each having a distinct intelligence, volition, power, &c., thinking such a supposition inconsistent with the proper Unity of the Godhead; which is doubtless one of the most obvious and fundamental doctrines of revelation.