The heresy took form when Arius heard his bishop, Alexander, give a sermon on the Trinity (always dangerous), and, unsatisfied with some of Alexander's explanations of how God was three persons yet one God, then wrote a letter critiquing his bishop (even more dangerous). At that point, the Church had already dispensed with Sabellianism (the heresy that Father, Son and Spirit are three different modes of the same person and not three distinct persons of the Trinity). Alexander's emphasis on the unity of the three persons reminded Arius of that other heresy, and he reacted to his bishop's teaching by questioning whether the Son and Father could both be eternal, equal, and of the same substance if the Son became incarnate. In other words, he asked, "How can God be one (monotheism) and identical with God's self (unity) if one of the persons of the Trinity became man?" Given God's unchanging nature, Arius concluded that the Son must not be eternal and, therefore, must not be equal with the Father. The seeds of controversy were sown, and, since Arius was a gifted preacher and teacher and the presbyter of an important church in an important city, his message quickly spread through the Church.
At that point, the Church didn't have any collectively defined Creeds...yet. But in 325 CE, in response to Arius' teaching, the first great ecumenical council was convened at Nicaea. Out of that meeting came the first Nicene Creed, which is found in the slide show video below. The text of the Creed shows that the early Church wasn't quite ready to define exactly who Jesus Christ was (and certainly wasn't ready to say much about the Spirit), but the Church was certain (or appeared to be) about what they didn't believe. Thus, the teachings of Arius and his followers were anathematized (see the end of the Creed). Actually, as so often happens in the church, it took a while for this orthodox position to take hold, and the consubstantiality of the Son and Father (and Spirit) wasn't secured until the next ecumenical council at Constantinople in 381 CE.
All settled, right? Well, maybe not. I still hear people say things about Jesus Christ that sound a lot like Arianism.
- "Jesus was a great teacher. He taught us how to live." That's Arianism. If Jesus Christ isn't fully God (equal and consubstantial with the Father), then he's no more than a wise man who had a lasting (though limited) impact on history.
- "Jesus came to show us how to live so that, following his example, we might go to heaven." Again, that's Arianism. If Jesus isn't fully God, then his death and resurrection don't do anything for us other than show us what true selflessness looks like. But we can't get to heaven by being nice (even nice enough to die for someone else). Jesus' death on the cross isn't an example to be repeated; it's a moment when all sin is put to death so that we might share in the new life of the resurrection.
- "God the Father sending his Son to die on the cross isn't love; it's brutal violence." Surprised that's Arianism? Well, if God the Father and Son aren't equal, then the Son becomes a subordinate demiurge who is subject to the whim of the Father. Instead, we believe that the will of God the Father is identical with the will of God the Son--they share the cross as the ultimate expression of selfless love.