To be honest only Catholics can truly claim to be Christians, when we look at the Holy Scriptures One Lord, one faith, one baptism. [Eph. 4, 5]. Protestants (this includes all denominations outside the Roman Catholic Church) are not apart of this fold, for they are not in unison with the Church that Christ Himself founded (Matth. 16, 15-19). If you are in a faith that contradicts the teachings of the Apostles and the Roman Catholic Church then you cannot claim to be apart of this “one faith.” We know that the Scriptures are hard to understand: *“*As also in all his epistles, speaking in them of these things; in which are certain things hard to be understood, which the unlearned and unstable wrest, as they do also the other scriptures, to their own destruction.” II Peter III, XVI, and that private interpretation will lead to disaster (2 Peter 1, 20). If Scriptures are clear to understand, why did Martin Luther and his imitators make new catechisms of Christian doctrine? Why do they fill the libraries with innumerable books of interpretations, explanations and commentaries? Above all, why have they any churches, where the Scriptures are explained, if the Scriptures are sufficient? And if there should be a church, why so many churches, of so many denominations, in every city and in every town? If there is a clear statement that in the Scriptures which all Christians should endeavor to put into execution, it is certainly the desire which Christ expressed in His prayer to the Eternal Father on the eve of His Passion and Death, “That they all may be one, as thou, Father, in me, and I in thee; that they also may be one in us; that the world may believe that thou hast sent me” (John 17, 21). What else do such words mean than that all Christ’s followers should first of all have the same faith?—“One Lord, one faith, one baptism.” (Eph. 4, 5).
It is a historical fact that the Church of Rome was always considered the first, the most important, and the most powerful Church in Christiandom. This was admitted in the first centuries and is so regarded today. She saved civilization and the Scripture. Her children outnumbered by far members of all other churches put together; and what is most important, they are bound in “One Faith,” as there can be but “one Lord, one faith, one baptism.”
“While they argue from the Scriptures, they make poor exhibition of the Scriptures… because truth cannot be found by those who do not know Tradition, for not through writing, but by the living voice, Tradition was delivered.” Saint Irenæus (born between 135 – 140 Anno Domini., Smyrna / died 202 Anno Domini., Lyons., disciple of Saint Polycarp)