After a long absence, I think it is time for the return of the good old theological debates. Many may not find them interesting, but in the past at least they were always respectful and thought-provoking. So, after a bit of discussion with Sigurd, I have set up this thread on the doctrine of the Trinity. There is no formal debate as such, but I will be arguing for it, Sigurd against.

Since this is just an informal discussion, I will make a few points to get the ball rolling. But firstly, I will explain what exactly my position is. The Trinity is the idea that there is one God who is manifested in three 'persons': the Father, the Son, and the Holy Spirit. God is not divided between these three persons; the fullness of God exists completely and indivisibly within each of them, and acts in various different ways through each of them. Below are some arguments which will hopefully get things going:

1. There is one God
Spoiler Alert, click show to read: 
I don't think that the monotheistic nature of Christianity is too contentious, so I'll just give two verses in support of this statement. Many such verses can be found throughout both the Old and New Testaments, which proclaim that there is one God, and no other gods beside him.
Hear, O Israel: The Lord our God is one Lord. (Deuteronomy 6:4)
And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord. (Mark 12:29)


2. The Father, the Son, and the Holy Spirit are all called God
Spoiler Alert, click show to read: 
While the scripture teaches that there is one God, there are many instances, in varying circumstances and contexts, where the Father, the Son, and the Holy Spirit are called God, and clearly viewed as being divine in nature. Below is an example of the Father being called God:
To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ. (Romans 1:7)
In the very first chapter of the New Testament, Jesus is referred to as "God with us", and that his coming was the fulfillment of the prophecies given in the Old Testament:
Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. (Matthew 1:23)
John's Gospel teaches that Jesus (referred to as "the Word") has always been co-eternal with God the Father, and that he existed long before he was manifested in a human body:
In the beginning was the Word, and the Word was with God, and the Word was God. (John 1:1)
When Jesus was manifested in the flesh, another of the disciples, called Thomas, still clearly worships him as God:
And Thomas answered and said unto him, My Lord and my God. (John 20:28)
Beyond the Gospels, Jesus is similarly called God in the Pauline epistles:
And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory. (1 Timothy 3:16)
The references to the divinity of the Holy Spirit are less obvious, but they are certainly there. Jesus himself notes that to blasphemy the Holy Spirit is even more perilous to our souls than to blaspheme his own name - blasphemy of course being an offence committed against God:
And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the world to come. (Matthew 12:32)


3. The Father, the Son, and the Holy Spirit share the same titles
Spoiler Alert, click show to read: 
As well as each being called God, titles which are given to God are also given to the particular persons of the Trinity. In other instances, particular titles which are in one place given a particular person of the Trinity, are in other places given to others. For example, the Father, the Son, and the Holy Spirit are in various places each called creator, an attribute which the very first verse of scripture gives to God:
In the beginning God created the heaven and the earth. (Genesis 1:1)
But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him. (1 Corinthians 8:6)
For by him [the Son] were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by him, and for him. (Colossians 1:16)
By his spirit he hath garnished the heavens; his hand hath formed the crooked serpent. (Job 26:13)
The scripture claims Jesus to be the alpha and omega, an attribute given to God as spoken of by the Old Testament prophet Isaiah:
Thus saith the Lord the King of Israel, and his redeemer the Lord of hosts; I am the first, and I am the last; and beside me there is no God. (Isaiah 44:6)
I [Jesus] am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty. (Revelation 1:8)
Similarly, Isaiah preached that God is our only Saviour, a title which is of course is central to Jesus mission on earth, and is granted to him repeatedly throughout the New Testament. Also, both God and Jesus are called Saviour in the New Testament scripture:
I, even I, am the Lord; and beside me there is no saviour. (Isaiah 43:11)
Neither is there salvation in any other [than Christ]: for there is none other name under heaven given among men, whereby we must be saved. (Acts 4:12)
To the only wise God our Saviour, be glory and majesty, dominion and power, both now and ever. Amen. (Jude 1:25)
Likewise, both God and Jesus are called Lord. God is often called Lord in the Old Testament, while both God and Jesus being called Lord even in the New:
Then they that feared the Lord spake often one to another: and the Lord hearkened, and heard it, and a book of remembrance was written before him for them that feared the Lord, and that thought upon his name. (Malachi 3:16)
For the wages of sin is death, but the free gift of God is eternal life through Christ Jesus our Lord. (Romans 6:23)
Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. (Matthew 22:37)
Again in a similar vein, both God and Jesus are claimed to be that "I AM". The Jews attempted to stone Jesus for saying such a thing which would make him equal with God - the grave sin of blasphemy. Of course, it is not blasphemy if Jesus himself is God:
And God said unto Moses, I Am That I Am: and he said, Thus shalt thou say unto the children of Israel, I Am hath sent me unto you. (Exodus 3:14)
Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am. (John 8:58)


4. The Father, the Son, and the Holy Spirit are separate persons
Spoiler Alert, click show to read: 
While all three persons of the Trinity are God, it is important to note that they are not merely names given to describe certain operations of God, or manifestations of God particular to time and place. Rather, they are each distinct persons which are and co-eternal and co-equal (see John 1:1 above). Certainly, it would not make sense for Jesus to pray to himself, yet he prays to the Father:
Then said Jesus, Father, forgive them; for they know not what they do. (Luke 23:34)
Also, while the title of 'God' is granted to both the Father and Jesus throughout the New Testament, they are continually referred to as distinct persons, a distinction which is maintained even when they are spoken of together:
To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ. (Romans 1:7)
Even in his resurrection and glorified state, Jesus remains distinct from the Father as he sits at the right hand of his throne in heaven, refuting the idea that Jesus was just a fleshly manifestation of God to carry out a particular mission:
Who [Jesus] being the brightness of his [God's] glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high. (Hebrews 1:3)
Also, Jesus clearly refers to the Holy Spirit as a distinct person, a "him", that is sent from the Father:
And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you. (John 14:16-17)


5. Each of these persons are fully and indivisibly God
Spoiler Alert, click show to read: 
An important aspect of Trinitarianism is the idea that the essence of God is not somehow divided between the three persons, as though 1/3 of God was in each of them. On the contrary, the Trinitarian position is that God is completely and indivisibly present within each of them. The scripture states of Christ:
For in him dwelleth all the fulness of the Godhead bodily. (Colossians 2:9)
Despite the distinct personhood of the Son and Father as mentioned earlier, Jesus says that they share a sort of unity of essence:
I and my Father are one. (John 10:30)
Similarly, the Holy Spirit is said to proceed from the Father (and, Western Christians would say, also the Son), and is somehow also of the same essence:
But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me. (John 15:26)


That is my position as an orthodox Christian. I am interested to hear what exactly Sigurd's take on things is, and what the various other Christian posters in here will have to say.