Why is Trinitarianism more correct than Binitarianism? (Armstrong heterodoxy, but probably much older)
Christian binitarianism asserts God is One – but argues a "twoness", the Son in a subordinate relation to the Father.
In Binitarianism the Holy Spirit is seen as
the same as the Son, as embodied in Him.
In
The Shepherd of Hermas, explaining the Son, an angel declares:
"6[59]:5 The Holy Pre-existent Spirit. Which created the whole creation, God made to dwell in flesh that He desired. This flesh, therefore, in which the Holy Spirit dwelt, was subject unto the Spirit, walking honourably in holiness and purity, without in any way defiling the Spirit..." (Parable V)
Early Christian binitarianism see the Spirit as a person of God's being, who lived in Jesus, or as Jesus's pre-existent, co-eternal divine nature – either way, Jesus and the Holy Spirit are one person, whereas in Trinitarian theology they are two.
The Council of Nicaea (325) opposed the binitarianism of Arius, but was unsuccessful in that various forms of 'semi-Arian' doctrines continued to emerge, until the Council of Constantinople (381), when the Trinity was affirmed.
Epiphanius of Salamis noted, “Semi-Arians ... hold the truly orthodox view of the Son, that he was forever with the Father ... but has been begotten without beginning and not in time ... But all of these blaspheme the Holy Spirit, and do not count him in the Godhead with the Father and the Son.”
Binitarianism effectively wasted away, and Trinitarianism emerged as the Orthodox theology.
Then ... a resurgence in America, with the emergence of various persons who are entrusted with visions, or revelatory insights into the meaning of Scripture, and who found effectively what are new religions under the banner of Christianity ... their beliefs are forms of Semi-Arian binitarianism.
Scripture mentions prayer to the Father, and to the Son, but the Holy Spirit is never prayed to nor worshiped in the Bible. In Revelations the Holy Spirit is not mentioned. Thus modern binitarianism concluded that this is because the Holy Spirit is not a distinct person, but the mind, character, power, and active presence of God.
Trinitarianism holds the Holy Spirit is a person, "another paraclete" (John 14:16) whom Jesus speaks of as a being in his own right (cf John 16).