Many Christians are stumped when it comes to explaining the doctrine of the Trinity. Even though the word "Trinity" doesn't appear in the Bible, it captures very well the teaching of the Bible regarding the nature of God. The biblical doctrine of the Trinity is comprised of the following seven truths:

1. There is one God (Deut. 6:4)
2. The Father is God (1 Cor. 1:3)
3. The Son is God (John 1:1; Rom. 9:5; Heb. 1:8)
4. The Holy Spirit is God (Acts 1:3-4; Eph. 4:30; Heb. 3:7)
5. The Father is not the Son (John 17:11)
6. The Son is not the Holy Spirit (John 15:26)
7. The Holy Spirit is not the Father (John 14:26)

These seven statements are illustrated in the following diagram:

This is certainly not meant to be a picture of God.  It is simply a way of illustrating the biblical teaching that there are three separate and distinct persons who share the one essence of God.  These seven truths constitute the doctrine of the Trinity.  If these seven truths are taught in Scripture (and they are), then the doctrine of the Trinity is a biblical doctrine.  It is necessary to believe in order to believe in the God of the Bible.