The Church is indeed universal. Peter was indeed the chief of the apostles. The Church is indeed the “pillar and foundation of the truth.” Much of the liturgy of the early Church Fathers certainly has a Catholic flavor. But when Jesus said, “I will build my Church,” did he mean a single visible instantiation centered in the city of Rome, or his universal mystical body (the Church militant, suffering, and triumphant), with many visible manifestations.
Much of the liturgy of the Early Church Fathers is the same liturgy used today. Of course there have been minor changes that do not change the effect, such as the language used, (from Latin only to the vernacular since VII). But the celebration of the Mass is essentially the same. Liturgy of the Word followed by Liturgy of the Eucharist, using unleavened bread and pure grape wine, using sacred vessels of fine materials, the prayers, the fact that the consecrated bread & wine is truly the body, blood, soul, and divinity of Jesus Christ which must be protected from any form of abuse, all of that is the same as from the beginning.
Did you know that every Roman Catholic church worldwide proclaims the same readings everyday? That Roman Catholic Mass is so similar worldwide that you can walk into any RC Church for Mass and know what is happening. Even if you don’t understand the language, if you are familiar enough with the Mass you can participate.
So when talking about the Catholic Church, the universal Church, it’s not just a set of beliefs we share, it’s not an independent church which happens to be Catholic. It is one Catholic Church with parishes worldwide so we, the body of Christ, can collectively participate in the Mass. I can go to Mass at St Peter’s Basilica or to a small station church of a parish with six families and I would still be participating in the same Mass.
The Scripture and the Church are his instruments .
This is it exactly!
But how do we know which visible manifestations of the Church are teaching the Truth?
We just have to go back to the beginning. We have biblical proof Jesus gave Peter the authority over the Church. We also know it was one church, not many. Even though there are scriptures using the plural of church at times, one has to take into consideration there were no other Christ based (Christian) churches in existence at that time. When scriptures refers to churches, they are referring to what we would now probably call diocese.
But even the Catholic Church confesses that the doctrines necessary for salvation exist within Protestant circles.
The Catholic Church acknowledges Protestant churches have truth in their doctrines but not complete truth. As I convert I know this to be the case. In my conversion nothing went against my Protestant background but I finally learned what had been missing from my faith, like I had only partial truth.