It may seem ironic, but the strongest argument that Protestants have against the Roman Catholic Church, stems from where we must agree the most with the Catholic Church.
Protestants must begin by affirming two truths: (1) sola scriptura: that the Bible is the inspired and infallible Word of the living God, the only objective rule of faith and worship; that the Bible alone -- not the Roman Catholic Church, the pope, nor church dogma -- is infallible; and (2) catholic orthodoxy: that the God who inspired the Bible has preserved of the correct understanding of His Word in history by means of catholic orthodoxy. By catholic orthodoxy we mean the unifying truths of the Church which are found in the four ecumenical creeds of the patristic Church (Apostles, Nicene, Athanasian and Chalcedonian).
Catholicity means literally "unity" or "universality." The term Catholic with an upper-case "C" is used to denote the Roman Catholic Church, while catholic with a lower-case "c" is used in creeds and confessions to denote all Christians. All true believers in Jesus Christ are, in this sense, catholic, because they hold to the univeral faith.
Orthodoxy means literally, "right opinion," and is expressed by the body of biblical doctrines systematized by the creeds of the early Church. Orthodoxy is the basis for unity among Christians of widely different beliefs and practices.
Affirming both sola scriptura and historic orthodoxy at once may seem contradictory, but in actuality, we cannot have one without the other. The Bible itself would not have been passed down to us today had the Church not assembled the canon of Scripture and faithfully preserved the texts written by God's apostles and prophets. We cannot accept the canon of the Bible unless we accept the authority of the Church council which received the canon.
Modern evangelicals are accustomed to hearing that the creeds are "Roman Catholic" and therefore bad. Therefore, the creeds are often neglected and not taught. But if we consider ourselves true Christians, then we must accept the creeds. We must also believe that certain biblical doctrines were faithfully preserved throughout the centuries by the Catholic Church, such as -- original sin; the Trinity; the human and divine natures of Christ; the virgin birth, the death burial and resurrection of Jesus; the Second Coming of Christ; the resurrection and judgment of the dead; and eternal heaven and hell.
In short, Protestants must agree with Roman Catholics in the area of historic orthodoxy.