The Trinity

Okay, so there's one God.

Thou believest that there is one God; thou doest well: the devils also believe, and tremble.
- James 2:19 (KJV)

Just like a cube has different sides, so God has different sides also.

The different sides of God (the Father, the Son, and the Holy Spirit) are often described as "living persons" having different roles.

PersonRoleMain character trait
The FatherCreator, sustains the UniverseLove
The SonGod in the flesh, forgiveness, judgmentMercy
The Holy SpiritComforter, teacher, counselor, helperTruth

Three persons, but one in Spirit. Working together in complete union.

For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.
- 1 John 5:7 (KJV)

One God expressed in three persons.

Note also that it says "in heaven". So there is one God, but in heaven (where God liasons with the angels) He is present as three persons.

The diagram below shows really well that there is only one God, but with three different sides to Him.

The trinity concept is not so strange when we look at our own lives. For example, I'm a father to my sons, a husband to my wife, and a friend to my friends. Those are all different sides of me. Different roles, but same me. The different sides of me obviously overlap, but hopefully you get the idea.

The Father giving His only begotten Son for the world is surely the greatest expression of Love there is.

For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
- John 3:16 (KJV)

Here's a great example from the Bible where all three sides of God are present:

And Jesus (the Son), when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God (the Holy Spirit) descending like a dove, and lighting upon him: And lo a voice from heaven (the Father), saying, This is my beloved Son, in whom I am well pleased.
- Matthew 3:17-18 (KJV)

And yes, the Holy Spirit is definitely a person.

Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
- John 16:13 (KJV)

Just remember that we can only go to the Father (Love) through the Son (Mercy).

Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
- John 14:6 (KJV)