The Holy Trinity
The dogma of the Trinity is the foundation stone of Christian theology. By it, the One God exists in Three Persons and One Substance. The doctrine is considered to be a mystery, that is, it can neither be known by unaided reason apart from Divine revelation, nor it can it be convincingly demonstrated by reasoning process after it has been revealed. But the mystery of the Trinity is not incompatible with the principles of rational thinking.
The doctrine of the Trinity has been explicitly taught in some passages of the New Testament, especially in the baptismal formula of St. Matthew's Gospel 28:19'... baptizing them in the name form 'in the name', used in connection with the Three Persons, it's linked with the other by the conjunction "and", clearly indicates the Unity in Trinity. In Christ's discourses after the Last Super, the mutual relations between Father, Son and Holy Spirit (Paraclete), are brought out with undoubted clarity. In all the Epistles, the Trinitarian formula is upheld frequently.
The New Testament teaching on the Trinity was taken by the Church and expresses in Creeds and Doxologies as well as in the confessions of faith on the part of martyrs. As heresies began to develop, the doctrine of the Trinity was the first to be put to scrutiny, and to be misunderstood and twisted by some heretics. The First and Second ecumenical councils, at Nicaea in 325 and Constantinople in 381 respectively, defined the dogma of the Trinity in it simplest but correct form. The co-equality and co-eternity of the Three Divine Persons were affirmed. The Persons differed only in origin; the Father is ungenerated, the Son generated by the Father, and the Holy Spirit proceeds from the Father. It should be noted in this connection that the procession of the Spirit "and from the Son" (see FILIOQUE) is a much later introduction on the part of the Roman Church which was effectively rejected by the treatise of Photios the Great and still continues to be one of the main dogmatical differences between the Eastern and Western Churches.