God chose to show Himself to the people of the Old Testament as the one, true, and personal God. Noah, Abraham, Joseph, David, Daniel, and all of the people of the Old Testament learned that God was real, that He was faithful and true in all His words and actions, that He loved them as a father loves his son, and that His love is everlasting.
Thus we read, for example: “For thou, O Lord, art good and forgiving, abounding in steadfast love to all who call on thee” (Psalm 86:5), and “As a father pities his children, so the Lord pities those who fear him” (Psalm 103:13). The mystery of the Holy Trinity would not be revealed until the New Testament.
God is the one, all-powerful Being, existing above and beyond man and his world. The true God is real. He becomes real for us when we come to know Him. To know God personally, we have to be very attentive to whatever He shows us of Himself. By making Himself known to us through Revelation, God gave Himself to us and showed us His will for the Salvation of all mankind.
God shows Himself in all of creation and history, but the Catholic Church is interested in the personal way in which God showed and continues to show Himself to His beloved human beings made in His image and likeness. This revelation is found in the Bible and in Sacred Tradition, and from it we learn what our life really is and how we must live it.