General Remarks
From the outset it has to be said that this is an extremely difficult doctrine. It is so difficult that no illustration is available to simplify it. To many people out with Christianity this doctrine implies polytheism, because they conclude that three persons mean three Gods. Historically, the controversies about this doctrine centred on the one hand on the protecting of monotheism, but in doing this there was the denial of the personal distinctions within the Godhead; and on the other hand failure to handle correctly the essential deity of the second Person (the Son), and the third Person (the Holy Spirit). Going back to the days of Origen, he regarded the Son as subordinate to the Father, and the Holy Spirit subordinate to the Son; and there were others with a similar view. The Arians denied the deity of both the Son and the Holy Spirit. They represented the Son as the first creation of God – a doctrine held by the Jehovah’s Witnesses. To preserve the unity of God the other two Persons were made to differ in rank thus discarding their consubstantiality. The three persons were also seen as a number of modes in which God revealed Himself.

At the Council of Nicea in 325 AD, the Son was declared to be co-essential with the Father. It took a further fifty six years before the deity of the Holy Spirit was dealt with at the Council of Constantinople in 381 AD. This attempt was not sufficiently adequate. There was official agreement that the Son was generated by the Father, and that the Holy Spirit proceeds from the Father and the Son. It was the work of Augustine that removed the elements of subordination, and was a complete statement. During the Post Reformation period this doctrine was again under attack. The Arminians regarded the Father as having preeminence over the Son and the Holy Spirit. To Schleirmacher the Trinity was just three aspects of God.

God as Trinity in Unity
God is Tri-personal and this is essential to His Being, and indeed explains the difficulty in illustrating this mysterious doctrine. W.G.T. Shedd says that there is a general self-consciousness of the Triune God as distinguished from the particular individual self-consciousness of each of the Persons of the Trinity. For in self-consciousness the subject must know itself as an object, and also perceive that it does. This is possible in God because of His Trinal existence. God could not be self-contemplating, self-cognitive, and self-communicating, if He were not Trinal in His constitution. (Dogmatic Theology, p 393).

Because of limited space it is necessary to restrict Biblical passages to just a selected few from the many that could be cited. How clearly is this doctrine stated in the Old Testament? It has to be said that the three Persons are not named in any single verse. The name of God, Elohim, is appealed to because it is plural. The plural is to be found in some passages and the following are examples: In Genesis 1:26, And God said let us make man in our image after our likeness; and in Genesis 11:7, Go to let us go down and there confound their language… There are of course many references to the promised Messiah throughout the Old Testament, and supposing no other passages in the Old Testament referred to His coming except the following, they would suffice: Isaiah 7:14, Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall be called Immanuel: and Isaiah 9:6, For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The Mighty God, The everlasting Father, The Prince of Peace.

In the New Testament Mark 1:11 accounts for two of the Persons, the Father speaking from heaven, and the Son; and in John 1:32-34 we have in that passage the Son and the Holy Spirit, with reference to the first Person, And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him. And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending and remaining the same is he which baptizeth with the Holy Ghost. Throughout the accounts by the four evangelists – Matthew, Mark, Luke and John – there are constant references to the Father and Son, and also to the Spirit. In passages such as Acts 2 we have the outpouring of the Holy Spirit; and in Acts 8:28 and Acts 11:12 speech is attributed to the Holy Spirit. In Ephesians 4:30 warning is given against grieving the Spirit; and in Mark 3:29 blaspheming the Holy Spirit is an unpardonable sin: But he that shall blaspheme against the Holy Ghost (Spirit) hath never forgiveness, but is in danger of eternal damnation. However, the only passage where the three Persons are named is Matthew 28:19, Go ye therefore, and teach all nations, baptizing them in the name of the Father and of the Son and of the Holy Ghost.

From this it follows that the above passages demonstrate that upon them is based a Tri-personal Godhead. Therefore in the Divine Being there is one indivisible essence and hence God is One in His constituted nature – There is but one God, the living and true only. (Question/Answer 5 Westminster Shorter Catechism based on Deuteronomy 6:4, and Jeremiah 10:10.)

By this doctrine is meant that there are three individual subsistences, or distinctions. It is relatively easy to speak of distinctions among human beings, but in referring to distinctions within the Godhead great care must be exercised. There is a personal self-distinction within the divine essence, and not a different essence that constitutes each Person. To John Calvin, Persons meant subsistence, and what distinguishes each Person are incommunicable properties, such as begetting which belongs to the first Person, the Father, begotten, to the second Person, the Son, and procession, to the third Person, the Holy Spirit who proceeds from the Father and the Son.

The uniqueness of the divine nature is that it can subsist wholly in more than one Person. The unity of the essence in person is regarded as the numerical unity. In regards to personal subsistence, ontologically the Father is first, the Son second, and the Holy Spirit third, but this does not for one moment suggest subordination. The succinct definition of the Godhead in the Westminster Shorter Catechism Question/Answer 6 states: There are three persons in the Godhead; the Father, the Son, and the Holy Ghost; and these three are one God, the same in substance, equal in power and glory.

Economic Trinity
The outward working is called the Economic Trinity. Regarding redemption, the planning is ascribed to the Father, the purchasing of redemption to the Son, and the application of redemption to the Holy Spirit. For this reason we see Father applied to the Triune God as the originator not just of redemption only, but also the work of creation and providence; Ephesians 3:14-15, and James 1:17. He is also Father in the relationship between the first and the second Person of the Trinity whereby God calls the latter His Son, because He is eternally begotten of Him; and the Son calls Him Father; references to this relationship can be found in John 1:14,18; John 8:54; John 14:12-13. He is also called Father in His theocratic relation to Israel in Deuteronomy 32:6, Isaiah 64:8, and Malachi 1:6. In the New Testament He is a Father to all who love him, and by the Spirit of adoption acknowledge Him to be so, Romans 8:15-17, and 1 John 3:1. Erring on this doctrine is a fundamental error. There is no doubt that most heresies involve erroneous views of the Trinity.