What is meant by, I believe in the holy catholic Church? The word “catholic” means “universal”. There is a universal bond uniting all who profess faith in Jesus Christ. That union with each other is more complete among those who are Baptized into Christ. Those who share the same Creed, Sacraments, and Liturgy (forms of worship) grow ever more fully united to one another. The Gospels and the rest of the New Testament Scriptures indicate that Jesus established on earth a Church (assembly) of believers (Mt. 16:18) and entrusted it to his Apostles. It is a defined, lived reality in the Acts of the Apostles and is seen in the letters of St Paul and the other N.T. Epistles. By the year 90 A.D. We see in early Christian writings that it was called “catholic” in order to distinguish it from other religious movements not founded by the Apostles. So Christians who profess a belief in the bond of believers in Christ and with one another can say that they believe in the holy catholic Church. (Holy because Christ is the head of the body the Church (Colossians 1:18). Many Protestant Christians, like Billy Graham for example, accept the Creed with this understanding.
Those who believe that the church is, as scripture says, the body of Christ, (1 Corinthians 12:27) can say that they believe in the holy catholic Church.
Roman Catholics additionally mean that they accept the claim of its governance by of the successors of the Apostles. In other words they are accepting the authority of Bishop of Rome, whom they hold to be the successor of St. Peter.
It is necessary to believe this in order to share the Sacraments of the Catholic Church.