There is hardly any truth (doctrine) that influences a man's religion, more than the answer to the question, "What is your God like?" Everything springs from your answer, absolutely everything. If your God is harsh, so will you be, if he is loving so the worshiper. The inspired prophets of the OT understood this and hence their constant rebuking of the nation of Israel for running after false gods. This is how the Psalmist descirbed the similarity of a worshiper to his or her 'god' Psalm 115:
Those who make them will be like them
and so will all who trust in them.
Christians worship a Triune God and that truth shapes everything about their faith. God has revealed himself as Three Persons who are God in themselves and yet who dwell in perfect unity. Father, Son and Holy Spirit. Here it is in three steps (no theological pun intended):
There is One God.
There are Three who are God
Yet there are not "three gods" for the Three are One
The finest simple explanation available is The Trinity by Stuart Olyott. Somehow the One God unfolds into three different Persons who all relate to one another in eternal love.
Out of this wonderful high and Glorious Mystery flow numerous implications - here are some of them:
Life Changing Implications of the Trinity
(1) The only way to come to God is by faith. Since God is transcendent, not only in his being (we can't understand the Trinity), but also in his works, his ways and his thoughts, we do not approach God as we do any one or anything else. These we can understand, they are below us, we might expect to have some dominance or control over them. We are the observer, they the subject. But God is above us, and the only way to approach him is by simple Faith, as a little child approaches his or her parent. There is no access to God through proud reason.
Apply - if you want to approach God, you have to humble yourself before his majesty right at the start. For our walk with God not only starts with Faith, it continues by faith till the end.
(2) Relationship is at the heart of God. Three Persons in blessed communion with one another. We catch a glimpse of this loving intra-divine fellowship in John's Gospel, between the Father and the Son.
Apply - relationships are actually at the heart of life. If relationships are going south we feel sad, if they are good we feel happy. That's the way we are wired, made in the image of God. And that's why relationships are the top priority of our lives.
(3) We can know God because he is personal. The reason we can have a relationship with God is because relationships come naturally to him. Why? Because from all eternity he has been in relationship with himself. A non-trinitarian religion that was monotheistic would have a God who was not used to any relationships and thus could hold out no prospect of us knowing him. That is why in Islam God is unknowable. Of course. He has never had any relationships, so why would he have one with us? He wouldn't know how to!
Apply - We can know God!
(4) Love comes from God. Now we understand what John means when he says that love comes from God. Love can only exist in a relationship with another. So a God who is not triune, cannot be loving. And indeed this helps us understand why Islam's Allah is so harsh. But a God who has dwelt in eternal loving relationship becomes the source of love - the one who has poured out his love on us in the sending of his Son. No Trinity, no love.
Apply - God is love
(5) Diversity can live together in unity. The Father is so different from the Son. The Father tells the Son what to say and do; the Son doesn't do that. The Son became a man suffered and died; the Father didn't do that. The Spirit was sent from the Father and from the Son, not the other way round. And yet they dwell in perfect unity, the Father and I are One, said Jesus. God loves diversity and declares it can live in unity.
- in marriage, the very different male and female can dwell as one
- in the church, Jew and Gentile, slave and free, male and female, can live as one
- even in creation different plants and animals all live together in unity
- in the bigger cosmos, large objects (ruled by Relativity) and small objects (ruled by Quantum Mechanics) dwell together in harmony
All of this is an expression of the glorious unity-and-diversity found deep deep within the Godhead.
Apply - accept diversity and work towards unity
Ever wondered why all Jehovah Witness churches are the same? They deny the Trinity, and without them even realizing, this denial has a deep deep impact on their church life. A one-god isn't into diversity and loves mono churches. By contrast the vast diversity of true Christian churches demonstrates the amazing Trinity which binds these churches together in love and worship:
“Holy, holy holy, Lord God Almighty
Early in the morning our song shall rise to Thee
Holy, holy, holy, merciful and mighty
God in Three persons, blessed Trinity”