Binitarianism is the rare belief that the Holy Spirit is not God while the Father and the Son are God. It differs from Arianism in that it affirms that the Son is God while disagreeing with trinitarianism by denying that the Holy Spirit shares equally with the Father and the Son the one divine nature. It reduces the Holy Spirit to God’s active force and makes him a created being. This viewpoint was common among the semi-Arians of the fourth century who were known as the pneumatomachi or those who fight against the Spirit. The leading spokesman for this group was Macedonius I, bishop of Constantinople. Eusebius of Caesarea and Origen could be classified as binitarians since they held that the Spirit is a created being though they often subordinated the Son to the Father (Khaled Anatolios, Retrieving Nicaea, 67; Eusebius, Ecclesiastical Theology 3.6.3; Origen, Commentary on John 2.6). Binitarianism was seen most recently in the beliefs of Herbert W. Armstrong and the Worldwide Church of God.
The reason why there are so few binitarians is because while they deny the deity of the Holy Spirit, they also deny the chief argument used against trinitarianism: the divine nature cannot be shared by more than one person or else this would lead to polytheism. While they are not trinitarians, they have no problem with God existing as more than one person. But very few of those we might classify as binitarians are true binitarians. They cannot consistently argue against trinitarianism while at the same time affirming that the divine nature can be shared by more than one person. This is why binitarians almost always hold to some form of subordinationism in their understanding of the relationship between the Father and the Son. They are not actually true binitarians, but we call them this because they refer to the Son as God even though there is almost always some qualification in their assertion that the Son is God.
While some scholars assert that the apostolic church fathers were binitarians and trinitarianism only developed later, there is ample evidence to indicate this is incorrect. One of the most common titles for the Holy Spirit in the writings of the early church is “Divine Spirit” which affirms that the Spirit is divine or God (Justin Martyr, First Apology 32; Dialogue with Trypho 7; Athenagoras, A Plea for the Christians 9; Irenaeus, Against Heresies 5.8.2). The trinitarian formula of Father, Son, and Holy Spirit is common throughout their writings and the Spirit is always closely associated with God. An example of this can be seen in Athenagoras: “Who, then, would not be astonished to hear men who speak of God the Father, and of God the Son, and of the Holy Spirit, and who declare both their power in union and their distinction in order, called atheists?” (A Plea for the Christians 10). The Spirit is viewed as the one who created all things (Shepherd of Hermas, Similitude 5.6.5). And only God created all things (Isa 44:24).