In the history of Christian theology, we often find that the church comes to its clearest understanding of the Word of God when it is forced to confront error. To sufficiently answer the heretics’ distortion of the Bible and its meaning, church leaders dig deep into the Scriptures and come up with vocabulary that is able to summarize all of what God’s Word teaches. When it comes to the doctrine of the Trinity, the battle with heresy played a particularly important role in the early church, and it was in confronting various heresies that much of the language we use to define Trinitarian doctrine was developed.
Since the doctrine of the Trinity requires us to affirm that in one sense God is one and that in another sense God is three, it is not too surprising that most Trinitarian heresies tend to emphasize God’s oneness at the expense of His threeness or His threeness at the expense of His oneness. This was certainly true during the first few centuries of church history. One of the earliest heresies emphasized God’s oneness so much that it had no room left for God’s threeness. This heresy, which is often called modalism, collapses the three persons of the Godhead into one person. In modalism, God is Father, Son, and Holy Spirit, but He is not simultaneously Father, Son, and Holy Spirit. There is no eternal fellowship between Father, Son, and Spirit; rather, at one point in history God was the Father, then He switched to being the Son, and now He is the Holy Spirit.
The church rejected modalism because Scripture clearly teaches that the three persons are eternal. Psalm 110, for example, has two of the persons speaking to one another, not one setting aside His identity and picking up another. Furthermore, that the Son can pray to the Father (John 17) indicates that the incarnation was not a matter of the Father exchanging His identity and becoming the Son. Otherwise, how could the Son speak to the Father during the Son’s earthly ministry?
A second major Trinitarian heresy in the early church was Arianism. Named for its chief proponent, Arius, this heresy said the first and greatest creation of God is the Son of God, by whom God made all other things. So, the Son has an exalted place in Arian theology over all creation. However, as exalted as the Son might be in Arianism, He is still a creature and not eternally God. Given that the full deity of Christ is clearly taught in passages such as John 1:1–18, it is not surprising that Arianism was finally rejected by biblical Christianity.