The mystery of the Holy Trinity is that there is in the one, true God only one divine substance, yet three divine Persons: God the Father, God the Son, and God the Holy Spirit. The Trinity is called a mystery of faith because it is a truth that cannot be known unless it is revealed by God, and it is a truth that we will not be able to fully understand.
Jesus Christ made known to us the secrets of the Kingdom of Heaven. The greatest of these is the secret of God Himself. Jesus taught us of the life of God. He said that there are three Persons in the one God and that each of these Persons are equal to the other. He also revealed the names of these three divine Persons: the Father, the Son, and the Holy Spirit.
The Holy Trinity is the central mystery of Christian faith and life.