Tradition
Tradition is something passed on and its passing on, like English "transmission", the act of sending and what is sent. Christianity is a matter of tradition; Scripture is part of tradition; older is better; apostolic succession for safeguarding tradition; Tertullian: Scripture "belongs" to the Church, so if there is a disagreement about Scripture it is really about tradition; but Tertullian left the Church to join the Montanists, so the debate was then about which church. Origen distinguishes between accepting tradition and speculating on the nature of truth, based on philosophy but Tertullian would not have it interfering with theology. Augustine's radical new thinking on the soul, the 'fall' and predestination became part of the tradition. The Church mitigated Augustine.
Vincent of Lerins, Communitory 9434) proposed that: Quod ubique, quod semper, quod ab omnibus (the faith is that which has been believed everywhere, always, by all) and this is how Scripture must be interpreted; but the Church is like the body, changing but the same and so it must: shape the rudimentary, consolidate the shaped and guard the ratified.
In the Ninth Century people believed that what they did was exactly the same as the Apostles and the few extant ancient texts had more authority than their authors had claimed; and a claim that there was an unwritten tradition from the Apostles that the Church could crystallise, e.g. Bonaventure on the Immaculate conception. The authority of tradition embedded in Scholastic method. Alleged that Medieval theologians put authority above Scripture but not true, notably of Aquinas.
The 'Middle Ages' ended when authorities were seen to have erred and when doctrine was seen as organic: The Donation of Constantine was spurious; constant copying had corrupted key documents; and the fall of Constantinople (1453) flooded the West with Greek scholars. Erasmus (1466-1536) ad fontes; Luther saw himself appealing to tradition; Scripture was part of tradition but the ultimate arbiter was the Gospel. Radical reformers, opposed by Luther, such as the Anabaptists, went much further and said anything not Biblical must be rejected. Calvin simplified in reverence to tradition. Reformers hoped to settle differences through a Council but Trent refused to argue and put oral tradition on a par with Scripture.
But modernity saw truth in the future: Protestantism claimed it was the religion for modernity which was simple and did not need traditional doctrinal 'clutter, ranking simplicity above tradition and Scripture; Roman Catholicism saw modernity as a bane with its loss of spiritual and temporal power, revolution, unorthodox teaching and countered with Ultramontanism with Pope Pius IX unilaterally proclaiming the dogma of the Immaculate conception in 1854 and in 1865 published the Syllabus of Errors attacking democracy and the secular state and in 1870 the First Vatican Council declared the Pope infallible.
Synthesis: Thus the gap between the two branches of Christianity was wider in the 19th Century than the 16th; and although Protestants claim the division was over the authority of Scripture it was essentially over authority itself.
John XXIII's Aggiornamento and Protestant neo-orthodoxy brought the two branches closer together but then postmodernism intervened. Theologians faced with the task of elucidating how the Gospel can be one in so many incarnations.