The third key doctrine of historical Christianity or Christendom (in addition to the Atonement and Incarnation) is the Trinity doctrine. The Trinity is the foundational teaching of so-called orthodox Christianity. It is putatively the mystery of God and the mystery of salvation (mysterium dei et mysterium salutis). This teaching is a mystery in that it is supposedly a product of divine revelation. In the words of Karen Armstrong, the triune Godhead lies "beyond words, concepts and human powers of analysis" (Armstrong 1993:118). God's triunity is utterly incapable of being understood by human reasoning.
a) The Trinity is not explicitly taught in the Old or the New Testament. Nevertheless, those who consider themselves orthodox Christians believe that there are subtle intimations of the doctrine in the Holy Scriptures of both Judaism and Christianity (Genesis 1:26; John 1:1). For example, the creation account recorded in Genesis depicts God uttering the words: "Let us make humankind in our image." Why is the Hebrew deity depicted as utilizing the plural pronouns "us" and "our"? Is the Genesis text suggesting that God is not a bare monad but a differentiated triad? Those who consider themselves orthodox Christians generally answer this question in the affirmative.
b) Despite such scriptural "intimations," however, the Trinity as a doctrine qua doctrine did not become a historical reality until 381 A. D. In that fateful year, the First Council of Constantinople (modern Istanbul) decided that the Father, Son, and Holy Spirit eternally have a "single Godhead, power and substance in three divine persons." In other words, God is ontologically (in his very being) triangular. Nevertheless, in what sense is the Christian God triangular?
c) A triangle by definition has three sides or angles. According to most logicians, triangularity is a necessary property of a triangle. Trinitarians similarly insist that God is necessarily Father, Son and Holy Spirit. The "Christian" God did not gradually come to possess three sides, as it were. He is immutable and atemporal (i.e., timeless), Trinitarians contends. God thus has always been "three-sided" in the estimation of orthodox Christians: he is eternally triangular.