Christians believe in one God, who is the Father, the Son, and the Holy Spirit.

The word trinity (trinitas) has traditionally been used when speaking about God and His holiness, though the Bible doesn't use the term. This article of faith is so important to the Church, that those who deny it haven't traditionally been considered as Christians at all. But what is it all about?

The purpose of this brief introduction is not to explore this important subject with the depth it would need; many essays on Christian faith written by church historical figures, e.g. an extensive work of Church Father Augustine, have already done it. However the presentation dealing with only the basics probably also comes in handy.

Starting point

From the very beginning, Christians have confessed the faith of the Old Testament: there is only one God. It is the confession of Israel and e.g. Paul the Apostle has a role in it.

"Hear, O Israel: The Lord our God, the Lord is one."
(Deuteronomy 6:4, ESV)

"For even if there are so-called gods, whether in heaven or on earth (as indeed there are many “gods” and many “lords”), yet for us there is but one God, the Father, from whom all things came and for whom we live; and there is but one Lord, Jesus Christ, through whom all things came and through whom we live."
(1 Corinthians 8:5-6, ESV)

While the whole New Testament clarifies this fundamental principle, it also talks about the Father, the Son, and the Holy Spirit.

"Then Jesus came to them and said, “All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.”
(Matthew 28:18-20, ESV)

"May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all."
(2 Corinthians 13:14, ESV)

So there is only one God, but still we talk about the Father, the Son and the Holy Spirit. The both perspectives must be kept in mind.

The Father is God, the Christ is God, the Holy Spirit is God.

I guess no-one who reads the Bible questions the deity of God. The New Testament also clearly teaches that Jesus Christ, the son of God, is God:

"No one has ever seen God, but the one and only Son, who is himself God and is in closest relationship with the Father, has made him known."
(John 1:18, ESV)

"We know also that the Son of God has come and has given us understanding, so that we may know him who is true. And we are in him who is true by being in his Son Jesus Christ. He is the true God and eternal life."
(1 John 5:20, ESV)

"For in Christ all the fullness of the Deity lives in bodily form"
(Colossians 2:9, ESV)

Already in the Old Testament, there is a mention about the spirit of God that He gives to his people:

"I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh. And I will put my Spirit in you and move you to follow my decrees and be careful to keep my laws.
(Ezekiel 36:26-27, ESV)

For example the Book of Ephesians explicitly states that the Holy Spirit is not a force comparable to gravity – otherwise it wouldn’t grieve – but a person. A quote from the Ephesians:

"And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption."
(Ephesians 4:30, ESV)

The Book of Acts tells us how Ananias and Sapphira lied to the Holy Spirit – in other words to God – and got the judgement they deserved.

"...you have lied to the Holy Spirit and have kept for yourself some of the money you received for the land?"
(Acts 5:3, ESV)

Thus, the Father is God, the Christ is God, and the Holy Spirit is God. And yet only one God exists. There is only one God, but He has three personifications.

The Father, the Son and the Holy Spirit clarify each other to their people

A young girl had just got saved and asked in confusion whether she should pray to the Father, the Son or the Holy Spirit. Her grandmother gave a simple but soul caring answer: “Pray to any of them. They will agree among themselves who should come and help you.” To support this splendid advice, let’s turn to the Bible.

According to God’s Son Jesus, we can’t know the Son without the Father.

"I praise you, Father, Lord of heaven and earth, because you have hidden these things from the wise and learned, and revealed them to little children."
(Matthew 11:25, ESV)

"Jesus replied, “Blessed are you, Simon son of Jonah, for this was not revealed to you by flesh and blood, but by my Father in heaven.”
(Matthew 16:17, ESV)

On the other hand, we can’t know the Father without Jesus:

“All things have been committed to me by my Father. No one knows the Son except the Father, and no one knows the Father except the Son and those to whom the Son chooses to reveal him.”
(Matthew 11:27, ESV)

Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me.
(John 14:6, ESV)

No-one can be touched by God without the Holy Spirit:

"...the Spirit you received brought about your adoption to sonship. And by him we cry, “Abba, Father."
(Romans 8:15, ESV)

"...and no one can say, “Jesus is Lord,” except by the Holy Spirit.
(1 Corinthians 12:3, ESV)

Thus the Father, the Son and the Holy Spirit glorify each other to us.

How can we begin to understand this?

In his excellent book ”Good News for Anxious Christians”, Phillip Carey quotes the seven basic statements by Augustine:

1. The Father is God
2. The Son is God
3. The Holy Spirit is God
4. The Son is not the Father
5. The Son is not the Holy Spirit
6. The Holy Spirit is not the Father
7. There is only one God

The statements are all consistent with the Bible. With the help of them we begin to understand the question: There is one God, with three personifications. We don’t have to try to dig deep into the essence of God since He is a great mystery, and in any case we’ll only understand a fraction of Him, a small part that has been revealed to us.

After comprehending these basic statements there is still a lot to wonder. Much more could have been said and also has been said ably in more comprehensive presentations. But I hope you got a good start with this one.