The word ‘catholic’ simply means universal. It is actually a good word rather than a bad word. The Reformers did not see themselves as separate from the catholic (universal) faith. In fact, they saw themselves as the true proclaimers of the historic faith of the Bible, embraced by all orthodox Christians throughout the ages of the Church. They were more than happy to align themselves with the ancient creeds of the Church and recited them in the Reformed worship services. These creeds affirmed the great essential doctrines of Christianity, including the Bible as the word of God, the Holy Trinity, the full deity and humanity of Christ, His sinless life and atoning, substitutionary death, as well as His physical resurrection from the dead. Though vehemently opposed to Rome’s aberrant doctrines they would describe themselves as truly catholic, though not of the Roman variety.
When Martin Luther and the Protestant Reformers opposed the Roman Catholic Church, what was being proclaimed was not some new doctrine never seen before. Instead, justification by grace alone, through faith alone in Christ alone was something the Bible proclaimed in both the Old and the New Testaments, and affirmed by many in the early Church fathers, even to men like Augustine (who was often quoted in the Reformers’ writings). The Reformation was a back to the Bible movement and marked a return to, and a recovery of, the one true biblical gospel embraced by all true believers in Christ. There was nothing new; nothing novel at all! This was the universal faith once for all delivered to the saints.
– John Samson