Arianism, named for it’s seminal proponent Arius (about 250-335 AD) generally is a denial of the triune nature of God and with it the denial of the deity and divine nature of Jesus of Nazareth as the son of God.
Arius held that it was impossible that God should have a son and therefore such language found in the apostolic writings must be interpreted as metaphorical or figurative in some way,
He taught in short that the son was a sort of deified creation of God the Father and occupied a unique place somewhere between God and man.
Arius did teach that the holy spirit was a person. He considered the Holy Spirit to be a person or a high-ranking angel, which had a beginning as a creature
Though Arius was famously condemned as a heretic at the council of Nicea in 325 AD, his influence continued in some sectors of the early church.
Key tenets of Arianism include:
God cannot have a “son.”
The “son of God” is not consubstantial, that is of one substance and essence with the father.
Therefore Jesus the son, was not coeternal and of equal divinity with the father.
The holy spirit is a person, distinguishable from the father and son.
The holy spirit is likewise with the son a created being and therefore also not coeternal and of equal divinity with the father.
Modern-Day Proponents of Arianism:
Most famously, the Watchtower Bible and Tract and Society whose members are known as “Jehovah’s Witnesses” are the closest modern proponents of Arianism. Though they deny the personality of the holy spirit altogether. They view Arius as a generally sound teacher who was unjustly treated by the early church.
A number of proponents of the modern unitarian universalist churches also share significant heretical ideas in common with Arian thought.