Why Do We as Catholics Believe That God Is Triune?

The Bible teaches that there is one God. Yet at the same time it calls three Persons (The Father; The Son, Jesus; and the Holy Spirit) God and describes all three in ways that pertain to God and not creatures. Because of this, the Church from the beginning has believed that God is one Being who subsists in three Persons. It has been said that someone living in a one-dimensional world, where there can obly be lines, could not imagine a square. And in a two-dimensional world, where squares exist, it would be very difficult to comprehend a cube. That's how God stands in relation to us. We must accept what He has revealed to us about Himself, even though it is very hard to understand. The Athanasian Creed (from the fifth century) provided a definition of the Holy Trinity.

...there is one Father, not three Fathers; one Son, not three Sons; one Holy Spirit, not three Holy Spirits. And in this Trinity none is before or after another; none is greater or less than another. But the whole three Persons are co-eternal together, and co-equal. So that in all things, as previously mentioned, the Unity in Trinity and the Trinity in Unity is to be worshiped. He therefore that will be saved must thus think of the Trinity. Furthermore, it is necessary to everlasting salvation that we believe also rightly in the incarnation of our Lord Jesus Christ.