The Catholic Church has remained faithful to Sacred Scripture and Holy Tradition over the past centuries. In the 1500s, the Protestant Reformation occurred, and the Council of Trent was created in response. This council made a strong counter-reformation, which was strengthened by Pope Pius V by making the church more disciplined and uniting it by promulgating the Tridentine Mass. Then, over the course of the twentieth century, many things changed and the modern era arrived. That is where modernism comes into play.
After the Ascension of our Lord into Heaven and the Descent of the Holy Spirit upon our Lady and the Apostles, the Church has faced numerous challenges, including persecutions from the Jews, the Roman Empire and other pagan nations. The demon used it to destroy the Church from the outside. Yet he also attempted to destroy the Church from within. There have been prior attempts to wreak havoc on the Church. Including critiques of corruption by a Catholic priest named John Wycliffe. But it was until 1517, when Martin Luther, an Augustinian Friar who wrote his ninety-five theses protesting against the sale of indulgences, became the wick of the Protestant Reformation.