When did the Roman Catholic Church become THE Roman Catholic Church? When would we say that they strayed from the true apostolic teachings for the doctrines of the medieval church?
1054 | Clearly up until July 1054 there was the Church, and that which was to be the Roman Catholic Church was the church of the Western Rite focussed around the western Patriarch, the Bishop of Rome. 1198 | Pope Innocent III, Profession of Faith prescribed for the Waldensians: "With our hearts we believe and with our lips we confess but one Church, not that of the heretics, but the Holy Roman Catholic and Apostolic Church, outside which we believe that no one is saved" 1215 | At the Fourth Lateran Council, "There is but one universal Church of the faithful, outside which no one at all is saved." Whilst we can argue that this was taught by the Fathers, including notably Clement, the force of the point here was to highlight to the Eastern Church their peril in being disconnected from the see of Peter. 1302 | Pope Boniface VIII in the Bull Unam Sanctam "We are compelled in virtue of our faith to believe and maintain that there is only one holy Catholic Church, and that one is apostolic. This we firmly believe and profess without qualification. Outside this Church there is no salvation and no remission of sins, … We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff." 1441 | Council of Florence "The most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the "eternal fire which was prepared for the devil and his angels" (Matthew 25:41), unless before death they are joined with Her; " My view is that the understanding of the nature of the Church changed markedly through this period, and that much of this is understood in the matters that led to the great schism, and the question of how much authority was to be found in the primacy of the Pontiff. Much of this found expression in the events of the 14th of February 1014, when the filioque clause was used in Rome for the first time.
We declare, say, define, and pronounce that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff. Golly, couldn't he have been a little more clear and precise?
The reason for the post over the 400 years following the great schism is that I think it shows the drift in thinking and self understanding of the Latin Church from being the Church of the West to being The Church. There was not single moment but rather a progression of thought. Couple with that was a drift from understanding the Church controlled by Grace to the idea of Grace being controlled by the Church. Whilst there were some great theologians in this period, and perhaps Aquinas stands tallest, in the period, and collectively referred to in the Homilies as the schoolmen, so we conclude that the light shone in the darkness.
Its birth spiritually was definite when Constantine reorganized the Church in circa. 313-316 A.D. (See The History of the Church, Eusebius, Chapter X: Constantine, Penguin Classics, pg 315). Constantine had re-erected the curtain Jesus tore down (Matthew 27:49-51), a separation of ecclesia and laity, when St. Paul said “so that, if I am delayed, you will know how each one must conduct himself in God's household, which is the church of the living God, the pillar and foundation of the truth,” (1 Timothy 3:15), and “But Christ is faithful as the Son over God's house. And we are his house, if indeed we hold firmly to our confidence and the hope in which we glory.” (Hebrews 3:6). Offically, The Roman Catholic Church is a true seperate entity and ecclesia (church) when it broke with Eastern Orthodoxy in 1054 A.D. However, you can seeds of the Roman Church’s importance in the Epistle to the Romans, “First, I thank my God through Jesus Christ for all of you, because your faith is being reported all over the world.” (Romans 1:8). Even then Rome had a prominence in the early church. Especially after the Jerusalem Church was smote by Herod Agrippa’s Persecution (Acts 12:1-3) and the Fall of Jerusalem in 70 A.D. to Titus and the Roman Armies. Naturally, with the Church of Jersualem gone, the Church in Rome took more prominence. It is a mistake however, to believe the Church of Rome in the epistle of Paul is the exact same as the one lead by Popes; the Papal Church would come later. For one, Paul wrote to the Roman Church salvation has certainty, “If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved,” (Romans 10:9-10 NIV) and Jesus gives us His righteousness, “But now apart from the law the righteousness of God has been made known, to which the Law and the Prophets testify. This righteousness is given through faith in Jesus Christ to all who believe. There is no difference between Jew and Gentile.” (Romans 3:20-21), while the RCC taught 2nd to Medieval doctrines of pennance, losing salvo (salvation) when sinnning after baptism (refuted by 1 John 2:1-2, 1 John 1:9), martyrdom to get to heaven (refuted by John 3:16-18, John 6:40, Philippians 3:9, Romans 10:9-20) and etc.