I grew up in an offshoot of the Worldwide Church of God, which was Binitarian rather than Trinitarian. What’s “Binitarian” mean? Essentially, we thought that God the Father and God the Son were God, whereas the Holy Spirit was an impersonal force rather than the third person of the Trinity.
An argument that I heard from a Binitarian relative of mine was that the controversies about the Godhead in the early days of Christianity concerned whether or not Jesus was God, whereas the Holy Spirit was not an issue. According to him, when the church officially declared that Jesus was God, it included the idea that the Holy Spirit was God as an afterthought. My relative’s implication was probably that the notion that the Holy Spirit was God (or even a personal being) was not a long-standing feature of Christian teaching prior to the Nicene Creed. My relative supported this argument with a scholarly entry in the International Standard Bible Encyclopedia.
As a result of what my relative said, I’ve had questions about the Holy Spirit in early Christian thought. Was the Holy Spirit believed to be a personal being or an impersonal force? And, if the Holy Spirit was long considered to be a personal being, did Arius address the alleged threat to monotheism that could result from considering the Holy Spirit to be God, since Arius had the same sort of concern about Jesus being God?
It turns out that the divinity of the Holy Spirit was also debated in controversies about the Godhead, and my impression is that the various sides agreed that the Holy Spirit was a personal being. Khaled Anatolios discusses this issue in his excellent (and readable) essay, “Discourse on the Trinity”, which is in The Cambridge History of Christianity: Constantine to c. 600.
Anatolios acknowledges that Arius did not focus his attention on the Holy Spirit, and that the Holy Spirit was an afterthought in the Nicene Creed. At the same time, Anatolios states on page 441 that Arius “certainly considered the Spirit to be a creature.” Anatolios also states that the Homoians and Eunomius “maintained that the Spirit…was subordinate to the Son and not an object of worship.” A group called the Tropici held “that the Spirit was created as an angel, the chief of God’s ‘ministering spirits’ (cf. Hebrews 1.14).” Against such ideas, Athanasius (who was prominent in the debate about Jesus’ divine status) and Basil of Caesarea affirmed that the Holy Spirit was God. So the various sides agreed that the Spirit was personal, but they disagreed about whether the Spirit was God.