A Christian confesses that God is One, yet God is seen in scripture as Three persons. Attempts to explain this has led to much controversy. The Nicene Creed of 325AD which we will look at later provides our Orthodox understanding. However, before the Nicene Creed, there were others who offered less than acceptable understandings. We must not look so much upon these as enemies of the faith, but see them as examples for caution. It seems their attempts were sincere, just sincerely wrong in their understanding of scripture.
How does a finite mind grasp the understanding of an infinite being whose otherness is so far from us we have nothing to relate to or compare Him to? We can only understand as far as His revelation reveals Him in scripture. Though He is infinite we can know Him by the scriptures, but no further. It has been the attempts to go beyond what is revealed that has led into error.
Monarchianism was one such attempt that can be broken down in two parts. One part being Adoptionism and the other Modalism. They both appeared in the second century and as Monarchianism does, they tried to explain the Oneness of God amidst what we know today as the Trinity.
Adoptionism would see Christ, not as God, but as a man who received the Spirit of God in a special way. This solves the mystery of the Trinity, but it leaves Christ as a man and not divine. This is unacceptable with Orthodox Christianity and had to be confronted.
Modalism was a form of Monarchianism that took another approach. They wanted to solve the mystery of the Trinitarian nature of God without doing damage to the divinity of Christ. It would hold that Christ was God, but only as a mode in that certain age. Modalism does not allow for the individual person of Christ along with the person of the Father and the Holy Spirit. This solves the mystery, but it leaves an incomplete Gospel and does damage to other parts of scripture.
Orthodoxy see God subsisting of three persons, the Father, the Son, and the Holy Spirit. All being of the same One essence, all being eternal, and all being divine. Adoptionism removes the divinity, Modalism ignores the personage.
Yet a greater threat was soon to arise, we will look at it next time.
For a much more in depth study please see the book by Harold O. J. Brown "Heresies"
May the Grace of God be with each of you,