Most Protestants believe in their own churches while at the same time claiming that the Church is not necessary as they have the Bible, their only authoritative source for their Christian faith. When they refer to The Church they mean an invisible union of believers as represented by the various Protestant sects. By invisible,  is meant that there is not a single human authority or headship over the body of believers. Each sect or church has its own head, whether it be a Bishop, a pastor or some lay leader. They deny the infallibility of the Pope when teaching in matters of faith and morals to the whole Church, to be believed, as an obligtaion of faith and practice. By doing so, they commit what they must intend to be an act of infallibility itself, or else they have no guarantee they are correct. Thus, while denying the authority of the Papacy as instituted by Jesus Christ, they arrogate to themselves that authority, in reality, although I don't think they have actually thought this through. In other words, denying that Christ's word to Peter is literally true [fundamental], they claim the right to interpret Christ's imperative to Peter in a non-literal manner, another irony because so many of them also say they accept the Bible as God's literal Word. Their source for the faith they uphold is based on "Sola Scriptura" or the Bible alone. This notion, now accepted by many Protestants as a dogma, did not come into being until the Protestant revolt. Until that time, all Christians held to the Catholic faith. It is also said that nowhere in the Bible is the name Catholic mentioned. Of course not, for there is only one true Church of Jesus, the one He established through His Apostles. The word Catholic refers to the universality of Christ's Church. Catholic, lower c, means universal. The capital form of the word was appended to the Church as it spread throughout the world. There is no mention of the Trinity, although the dogma of the Holy Trinity is very present in Scripture. No Protestant, outside of the percularities of Mormonism, denies the concept of the Holy Trinity. The same with the date for Christmas. These aspects of the Christian [Catholic] faith come from Sacred Tradition.

www.catholictradition.org/Tradition/necessity-church.htm