What is meant by saying that God is a trinity?

Answer 1

The Mystery of the Trinity refers to the fact that God is The Son, The Father and the Holy Spirit, the three of them at the same time, and it's a mystery because there is no way to figure out why or how is that. This is because it was added in by the church fathers and its so unbelievable that it is called a mystery.

Answer 2

Saying that God is a trinity means that there is only one God, but He exists in three persons: the Father, Son, and the Holy Spirit. They are unique aspects of the same God. It is a difficult concept to be comfortable with and is one of cornerstones of Christianity.

Answer 3

Great saints have written volumes to try and describe the mystery of the Most Holy Trinity. When I was in school it was described this way to me. God the Father, unlike you and me, has only One Idea. Of course that One Idea encompasses everything. We living in time have many ideas in sequence. Time, being a measure of change, is meaningless to God, Who just is, and does not change. God's One Idea, we call the son, the Word, or Jesus Christ. The love that exists between the Father and the Son is the Holy Spirit. All eternal having no beginning or end.