God, by nature is a Triune Being or Trinity. The Trinity, simply stated, is as follows: The Bible teaches that there is one eternal God who is the Creator and Sustainer of the universe. He is the only God that exists. Within the nature of this one God are three eternal persons, The Father, the Son, and the Holy Spirit. These three persons are the one God. The Trinity doctrine is based on what the Scripture says concerning the nature of God.

One God

A point that the Scripture clearly establishes is that there is only one God who exists:
Deuteronomy 6:4 Hear, O Israel: The LORD our God is one LORD: 1 Timothy 2:5 For there is one God, and one mediator between God and men, the man Christ Jesus; Isaiah 43:10 Ye are my witnesses, saith the LORD, and my servant whom I have chosen: that ye may know and believe me, and understand that I am he: before me there was no God formed, neither shall there be after me. These passages and many other Biblical statements make it clear that there is only one God.

Plurality Of Persons

Even though only one God exists we are told in Scripture that there is a plurality of persons in His being or nature. God said Genesis 1:26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth. God also said: Genesis 3:22 And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:

These verses allude to God's plurality of persons. We know that God could not be talking to angels here for angels did not, nor could they, help God create the universe.
This plurality, however, is not the same as a split personality or insanity. There are three separate personalities within the nature of the one God.

God The Father

The Bible talks about a person who is designated as the Father. This person is called God. Isaiah the prophet wrote:

Isaiah 64:8 But now, O LORD, thou art our father; we are the clay, and thou our potter; and we all are the work of thy hand.

Jesus taught His disciples to pray: "Our Father in heaven" Matthew 6:9

When the Apostle Paul wrote to the Christians in Galatia, he began his letter,
Galatians 1:1 Paul, an apostle, (not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead;) God is called the Father in both the Old and New Testaments.

God The Son

The Bible records the existence of a second person who is distinct from the Father and who is also called God. This person is Jesus, God the Son. John the evangelist wrote:
John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God In another place John wrote: John 5:18 Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God. The New Testament makes it clear that Jesus is God.

God The Holy Spirit

There is a third Person revealed in the Bible who is different from both the Father and the Son. He is known as the Holy Spirit. The Holy Spirit is also referred to as being God.
Acts 5:3,4 But Peter said, Ananias, why hath Satan filled thine heart to lie to the Holy Ghost, and to keep back part of the price of the land? 5:4 Whiles it remained, was it not thine own? and after it was sold, was it not in thine own power? why hast thou conceived this thing in thine heart? thou hast not lied unto men, but unto God.

To understand the doctrine of the Trinity key facts must be kept in mind:

1. The Bible teaches that there is only one God.

2. Biblical statements allude to the plurality of persons within God's nature.

3. The Scripture also says that there are three distinct persons, The Father, the Son, and the Holy Spirit. Each of these three persons are called God.

4. Since there is only one God and since the Father, the Son, and the Holy Spirit are all called God, the inescapable conclusion is that these three persons are the one God. This is the doctrine of the Trinity.