Today, the Trinitarian concept of God is the orthodox view for nearly all of Christianity.  Its development is fascinating, beginning with the desire to distinguish Christianity from paganism, by having only one God, like Judaism.  The concept was fine tuned with the idea of Jesus being both God and man, but distinguishable from God the Father.  Finally, with the Nicean Creed of 325 AD, convened to keep both the Roman Empire and the Christian Church from fragmenting, the Trinitarian concept was solidified to mean:  God is comprised of three persons, God the Father, God the Son and God the Holy Spirit, but of one undivided substance, a complete understanding and explanation of which remains a mystery.