Arius served as a presbyter at Baucalis, one of the parish churches of Alexandria, under Bishop Alexander, and taught that only God the Father is unbegotten, without beginning and without source. The Son, in Arius's account, is the Father's first and greatest creation, brought into being before all ages to serve as the agent of the rest of creation, but not co-eternal with the Father and not of the same substance. His own summary line, preserved only because opponents quoted it to condemn it, was blunt: there was when the Son was not.
The stakes were not abstract. If the Son is a creature, however exalted, then what unites humanity to God in Christ is itself something less than full divinity, and salvation rests on shakier ground than the church at large was prepared to accept. Bishop Alexander deposed Arius locally around 321, but Arius had allies among influential bishops in the East, chiefly Eusebius of Nicomedia, and the dispute spread through the empire's Christian communities quickly enough that the emperor Constantine, only recently converted and eager for a unified church, called a general council to settle it.
Almost nothing survives in Arius's own voice. His letter to Eusebius of Nicomedia and fragments of a verse work called the Thalia, meant to popularize his teaching in a form ordinary people could sing, are preserved only inside the writings of the opponents who quoted them to refute them, chiefly Athanasius, who spent much of his career as Alexander's successor prosecuting the case against him.