Arianism is the doctrine of the priest and ascetic Arius, died in the 4th century. This thinker was opposed to the Holy Trinity, since it rejected the divinity of Jesus Christ.
To understand what Arianism is, it is important to understand the idea of consubstantiality. According to the Catholic religion, the three divine persons are consubstantial: that is, they share the same substance. For Arianism, on the other hand, the second person (the Verb or the Child) lacks consubstantiality because the Son is subordinate to the Father.
This is linked to the fact that Arianism interprets different passages of the Bible to hold that the Son was created by the Father, so he did not exist forever. In this way, Arianism was left out of the christian orthodoxy.
In 325, the First Council of Nicaea stated that Arianism was heretical. However, ten years later, Arius was exonerated in the First Council of Tire. Years passed and, after his passing, Arius was declared again heretic in 381 (First Council of Constantinople).
Amid these theological debates and sanctions, Arianism maintained its popularity and several Roman emperors followed its doctrine. Over time it became an arduous task to reconstruct the thinking of Arius because his job was lost.
It is important to point out that, despite everything, the scope of Arianism reaches the present time. The Socinianism based on ideas of Fauso Socino and the movement of Jehovah’s Witnesses are often considered successors to Arianism for antitrinitarian postures.