From the late 350s, as the Son's full divinity became clearer, new questions arose about the Holy Spirit. Was he, like the Son, fully divine, "of the same substance" as the Father and the Son?
Athanasius addressed these questions in a letter to Serapion in 359. He defended the full divinity of the Spirit using the same principles that he had used to defend the full divinity of the Son. In this way, the whole subject of the Spirit clarified and deepened the church's understanding of the key issues in the Arian controversy.
Athanasius's three key principles were:
1) We must collect the sense of Scripture
2) There are no degrees of divinity
3) Only God can give grace