Arianism is usually summarized as "Jesus wasn't God," which is close enough to be useful and wrong enough to be misleading.

What Arius Actually Taught Arius held that the Son was a created being — the highest of creation, but not co-eternal with the Father. "There was a time when he was not," in Arius's own phrasing, was the crux of the dispute.

Why It Was Rejected Opponents, led by Athanasius, argued this made the Son a creature rather than truly God, undermining the entire logic of salvation as understood by the early Church. The Council of Nicaea in 325 AD responded with the term homoousios — "of the same substance" — to rule the Arian position out explicitly.

Why It Kept Resurfacing Arianism didn't die at Nicaea; it spread through the Gothic tribes and remained a live political force for another century, which is why Athanasius spent so much of his life in exile defending a single word in the Creed.