Why do Catholics believe that God is three Persons, called the Holy Trinity? How can God be three Persons and still be one God?

Catholics believe there is one God consisting of three distinct and equal divine persons - Father, Son and Holy Spirit - because on numerous occasions God has described Himself thus. The Old Testament gives intimations that there are more than one Person in God. In Genesis 1:26, God says, "Let us make man to our image and likeness."

And in New Testament, God reveals this doctrine even more clearly. For example, at the Baptism of Jesus Christ, the Holy Spirit appeared in the form of a dove, and the voice of God the Father was heard:"This is my beloved Son, in whom I am well pleased."(Mat.3:16-17). In Mat. 28:19, God the Son commanded the Apostles to baptize 'in the name of the Father, and of the Son, and of the Holy Spirit."

Father is God - (Is. 45:5)

Son is God - (Heb. 1:8)

Holy Spirit is God - (Acts 5:3-4)

"for these three are one"- (1 Jn. 5:7)

Three divine Persons in one Godhead may be incomprehensible to the human mind, but that is to be expected. We can testify ourselves as to the feasibility of God's triune make up by considering various realities. The triangle, for example, is one distinct form with three distinct and equal sides. There are many physical trinities on earth, therefore a Spiritual Trinity, who is God in Heaven, is not against human reason - it simply above human reason. :o