You mean the faith-beliefs of the Catholic Church? It’s kind of circular reasoning: the Catholic Church is the one true church and submitting to godly authority is something both Catholics and Protestants both claim to believe in, so why won’t Protestants just stop being so stubborn and submit?
The answer is that we have not been convinced by biblical proof that the Catholic Church is as infallible on all doctrinal issues that it claims to be. While Catholics would respond ( I suppose?) that it is not the Christian’s place to challenge church authority, Protestant history shows us that we see things differently. From Luther to modern times, Protestants have taken seriously the Christian’s responsibility to test all things. This is a responsibility of judgment, and church authorities are not exempt from having their teachings and actions judged by those under them simply because they are in positions of teaching authority.
Simply put, the Catholic Church, like all human organizations, can err. Since it can err in its teaching and its actions, nothing it or its agents teach should just be accepted without first having been found in accordance with the Scriptures–which teach no error.
Protestants have found error in Catholic Church teaching and have felt the need to correct those errors. Not being able to do that in the Catholic Church, it was sadly found necessarily to separate and reform the Church.
And yes, this process has continued and now we have many Protestant denominations–it’s not a great situation but it’s a lot better than being forced to remain subservient to ecclesiastical authorities in which you have no confidence.
Of course, truth is essential. Christ is the Way, the Truth, and the Life. Jesus Christ is truth, and there can be no grace without truth. The question is who or what defines the truth of the Christian faith?
Yes. Protestants believe in only one Church and one faith.
Yes, I’ve been told by Catholics that this mandate was only given to apostles (and now bishops). I’ve always found that interpretation odd. I was always taught and still am convinced that this commission was given to the whole church, not just the apostles.
By “maturing”, I mean growing into the unity of the faith. We aren’t quite there yet (and it can be argued that we never fully were after the Apostolic Age) and thus we have more maturing to do.