WARNING: This post will be written as simply as possible. BUT you probably still won’t understand what I’ve written!
Trinitarianism is the Christian understanding of God as three persons, known as the Trinity. These persons are: 1. God the Father, 2. God the Son (Jesus Christ) and 3. God the Holy Spirit. So, Christians believe that God is one being , who is worshiped as Father, Son and Holy Spirit, each of these are equally God.
It could be said that: God is one “What” and three “Whos” (Olsen, 2002, Pg. 147)
It is worth pointing out that there is no one illustration which does the concept of the Trinity justice. However:
Just before playing the Rachmaninov 2nd Piano Concerto in C minor, a concert pianist sits at the grand piano on the stage of the Royal Albert Hall in London. The pianist plays the note “A” on the piano for the orchestra to tune to. The pianist plays the note just once, so the orchestra hear just one sound.
Before the concert, the piano was tuned. The tuner, having stripped back the piano to its workings went to tune each individual note. He gets to the “A” note, like he has the 87 previous notes and finds three strings, each he tunes to the correct pitch. When the pianist plays the key on the piano, one hammer strikes three strings. These strings are of equal importance and three strings are required to make the characteristic sound of the piano.
Each string is Father, Son and Holy Spirit, the one note sounded is God.
Christians often refer to the Triune God to represent that the one God is made up of the Trinity. It is this Triune God who created In the beginning and who’s character is described in previous posts: Immutability and Impassibility, It is ‘all’ about the ‘Omni’ and Transcendence and Immanence. Monotheism is the belief in one creator God.
Other views on the Trinity:
Modalism is the belief that the Trinity are not three distinct persons, but is one person acting in different roles. God can only be in one role at a time. Image watching a Punch and Judy show; the audience see, Punch, Judy and the Crocodile, but they are just played by one puppeteer. The puppeteer can only give voice to one puppet at a time.
Subordinationism is the belief that God is the Father and that the Son and the Holy Spirit are Subordinate (or of lower rank) to the Father, but are still divine. The Father is the Field Marshall, whilst the Son and the Holy Spirit are both Generals. Christian understanding of the Trinity holds the view that Father, Son and Holy Spirit are equal, i.e. all hold the rank of Field Marshall.
Adoptionism is a version of Subordinationism; it is the belief that Jesus was not the true Son of God, rather he was a mere prophet and Messiah.
Jesus was adopted by God as His Son. Picture the scene: God is Daddy Warbucks in Annie and Jesus is Annie. Warbucks is not Annie’s true father, but Warbucks adopts her and she becomes his heir and lives with him. Christians believe that Jesus, the Son was with God from the beginning, known as The Word; therefore Adoptionism is not compatible with Christianity.
Arianism is also a version of Subordinationism. It is the belief that Jesus Christ is not God, or equal with God. Arianism holds that Jesus was created to live on earth and that He has not been since the beginning. Arianism is not compatible with Christianity for similar reasons as Adoptionism.
Tritheism is the belief the Father, Son and Holy Spirit are three separate Divine beings or gods. This is not compatible with Christianity which believes in monotheism.