"How do you explain the Trinity to a young believer?"
Forgive me if I play off the popularity of the "For Dummies" books. Their great success is a hopeful indicator that we don't mind admitting our ignorance on a number of subjects, including the doctrine of the Trinity.
'Trinity' comes from the Latin word, 'Trinitas,' meaning a triad or the number 3. The Trinity is the Christian teaching that there is One God consisting of three different yet equal beings: the Father, the Son, and the Holy Spirit (the "Godhead"
The Bible teaches there is but one God. "Hear, O Israel: The LORD our God, the LORD is one" (Deut 6:4). The Hebrew word for "one" means "united." Therefore the Trinity may be compared to the "United" States of America—one nation consisting of 50 states, unified by a common purpose. Likewise, the Trinity consists of three persons united by common purpose. Just as our 50 states possess unique qualities, but are "one" in common bond, so the 3 persons in the Trinity are one God.
The Trinity may also be explained by using the analogy of water. Water exists in three forms: a solid, a gas, and liquid. Each are different forms of the same substance. (One humorous analogy compares the Trinity to cherry pie, which is cut into separate pieces, but runs together underneath!)
We have trouble with this concept because we’re trying to comprehend a Being who is simultaneously One God and yet three Persons. We’re three-dimensional beings trying to fathom a four dimensional universe. Human reason can only take us so far. The Bible teaches that God's ways and nature are "unsearchable... beyond tracing out" (Rom 11:33). Where our understanding stops, our faith must lead us on.
Christians most assuredly do not believe in three Gods. That's polytheism, a false religion. Neither do Christians believe that "One God" means that God manifests Himself, at various times, as either Father, Son, or Holy Spirit. That's called Oneness Theology, and it lies outside the bounds of orthodox (correct) biblical teaching. (Note: Use the Internet to search for "Athanasian Creed." It’s an excellent primer on the Trinity.)
The Father, Son, and Holy Spirit together work for our redemption. They share a common resolve to save us. The Trinity, therefore, is not just a doctrine to believe; it describes a marvelous, saving God for us to worship and adore.
--Pastor Scott (www.askpastorscott.com)