Catholicism is often called a denomination of Christianity. We hear and read it, many Catholics even use the term, but this is contrary to what Jesus Christ defined as Church, the only one He founded on Peter.
Denomination in the religious sense describes a group that, along with others like it, forms part of a bigger, whole body. It indicates there is more than one version of a particular teaching and taken together in sum the fullness of truth can be found.
But when Christ built His Church upon Peter, He willed it to consist of believers united in one faith under His revealed truth, the Gospel. He prayed to the Father they will be one in His Mystical Body and not separated or divided by different versions of His teachings.
These disciples of Christ became known as Christians after His death and Resurrection. It was not a term they used to identify who they were, but given by others to tell them apart from other Jews who were followers of Judaism.
To make this distinction even clearer, this Church of Christians was also called Catholic (katholikos in Greek), meaning universal, because unlike Judaism which was for the Jews only, the Gospel was for all nations and races.
It is a command of Christ to his disciples in Matthew 28:19 – “Going therefore, teach ye all nations; baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.”
The term Catholic was also used to differentiate true Christians from heretics who contradicted what the Apostles taught. But the only surviving evidence of its description of the infant Church is found in the epistle of St Ignatius of Antioch (a disciple taught by the Apostle St John) to the Smyrnaeans in 107AD just before he was martyred. He wrote: “Where the bishop is, so is the Catholic Church.”
The one and only Church that Christ founded is, therefore, Catholic and if this is the case then She cannot embrace different versions of Christ’s Gospel. This means that Catholicism cannot contradict itself by saying it is only a part of the True Church and therefore a denomination of it.
But the term “denomination” can be appropriately applied to the different groups of the Protestant Reformation of the 16th century. It was triggered by Martin Luther who was disillusioned by the many abuses among the clergy and led others to break away from the Catholic Church.
Jean Calvin (Calvinists and Presbyterians) and Huldreich Zwingli (Anabaptists) followed him and each separately formed their own Protestant communities. King Henry the VIII was next with the Church in England (its adherents called Anglicans) breaking away from Rome and from among them John Wesley spawned the Methodist movement.
All were products of the protest against the Catholic Church, but each of them differed in various degrees to the doctrines that Martin Luther espoused. For this reason, they -- and the more than 30,000 groups that broke away from them since then, including the Baptists -- can rightly be called denominations of the Protestant Reformation.
And because they rejected many of the doctrines that Christ handed His Apostles, they possess only part of the revealed Truth.
On the other hand, despite the many difficulties with the clergy in the past, the Catholic Church handed down untainted and faithfully all that Christ taught through the successors of Peter, as He guaranteed it will be so under the Holy Spirit’s guidance until the end of time.
And since the Catholic Church is not a part of the Truth but the fullness of it, She cannot be a denomination of Christianity but, in essence and whole, Christianity itself.