Christianity is a monotheistic religion. This means that it only has one God. Some religions (like Hinduism for example) are polytheistic meaning that they believe in many gods which have different roles and purposes. Christianity declares that there is only one God. Here are some of the Christian God's qualities:
Although Christianity only has one God, He is seen in three ways. These are called the Father, the Son, and the Holy Spirit (sometimes referred to as the Holy Ghost). Some people struggle with the idea of one God being seen in three ways, but I like to imagine it as a triangle. The three sides are all their own lines and yet part of the same shape. (See my prism idea below). Another way of looking at it is to think of how one person can be seen in three ways (or more than three!). For example a woman could be a mother, a daughter and a sister. She is still the same woman yet she is viewed differently from different angles. It's not a terribly great example but I hope you can now see that one being can be known in three very distinct ways.
All three aspects of God have always existed though they have played their parts in different ways at set times. Each part of the Trinity is explained in more detail below:
The Father is the Creator. He made the universe and all that is in it. He is the God of the Old Testament and is often called LORD.
Jesus Christ is God the Son. He has always existed, and always will exist like the Father. God came to earth in human form in 4BC to show people how to live. Prior to this God's followers had been Jews. They respected the authority of the Old Testament and loved God as their creator. However, throughout the Old Testament there were prophesies that God would send a Messiah so that people could come to know God personally. This happened with the miraculous birth of Jesus over two thousand years ago. Jesus was the fulfilment of prophesies in Scripture and came so that people could know God personally. He explained who God is and demonstrated God's love and power through his teachings and miracles. His purpose was to put people right with God. Because people are not perfect but God is and cannot tolerate evil forever, He cannot accept sinners into his heavenly Kingdom. Yet because God loves us He wants to know us personally. To bridge the wide gulf between the perfect Father and us, sinful humans, Jesus (who, being God, was perfect) became human so that He could know us in a way we would understand and so that He could suffer. Jesus, though completely innocent, took all the pain and suffering which we deserve when he was crucified. Temporarily the Son was cut off from the Father as a way of paying the price for our sins.
Some question how Jesus can be both fully God and fully human. It is a concept that the finite human mind cannot fully fathom. To understand it completely we'd have to be God who transcends the laws of reason we know. Jesus was fully divine because He was from God, and was indeed part of God, and yet he took on human appearance and character.
The Holy Spirit is often the most neglected part of the Trinity. We often hear of the Father and of the Son but the Spirit is what is sustaining Christianity now. The Father and Son still both exist (Jesus was brought back to life after his death because, being sinless, death could not hold Him) and then he ascended* into heaven. The Holy Spirit is the unseen part of the Trinity which speaks volumes in the world today. It lives within those who accept Jesus as their Saviour. It is the Holy Spirit which is influencing me as I write this now!
*This does not mean he went in an upwards direction as many imagine; it means he went to a higher, more important place (i.e. heaven).
I know that the Trinity is a strange concept to grasp. As I said earlier, it cannot be fully understood by us because we're only human. But here is a way I like to think of it:
Imagine God is a hollow triangular-based prism (bear with me!):
Suppose each part of the Trinity was a rectangular face so that the Father was one face, the Son another and the Holy Spirit the third:
This is one shape and yet from different angles we see different parts. God can't be known intimately. He is incomprehensible and far too great for us to relate to. But God wanted us to know Him, so He tore Himself apart (even though we don't deserve it, and He didn't have to):
This was so we could know God intimately. We can now know God on a personal level:
Jesus conquered death and after the Ascention, he was reunited with the Father so that the Trinity was one again. However, the way still stands open for us to come and know God. Yet because we're not sealed off from the outside, Satan can still get in and tempt us with evil:
At the end of time as we know it God will judge us. Those (living or dead) who have accepted Jesus as their Saviour have got their places in Heaven safely booked for them. God will forever let those people know Him intimately and they will be safe from evil and temptation:
OK, so there are problems with this idea of God being a prism, but I hope it helps you understand the Trinity a bit better if you found the concept a hard one to grasp!