A common misunderstanding among Catholics is that "Protestants" (a term which I find highly misleading, but I'll use it for sake of clarity) believe the Gospel went extinct almost as soon as the Apostles died, and that it didn't get restored until Martin Luther came along. They also argue that since the Catholic Church was (allegedly) the only "real" Church for the first 1,500 years, it must be true and all "Protestant" denominations are just new inventions that were nowhere to be found for most of Church history.
The idea that "Protestants" believe the Gospel needed restoring is false. I don't know a single Christian who believes there was a point in history in which the true Gospel perished from the Earth. There is a religion that does believe that, i.e. Mormonism, but it is common knowledge outside of the Mormon Church that Mormonism is not Christianity. It is not even logical to call them Protestants.
The actual belief "Protestants" hold is that there were non-Catholic Christians, and even Christians within the Catholic Church, just as there are today, since the Catholic Church emerged. It's just that the Catholic Church quickly became a political superpower that literally had the legal authority to tie dissenters to a stake and set them on fire. The Catholic Church is notorious for having both burned books it disagreed with and killing people it deemed to be heretical. The mere fact that the Catholic Church killed those it deemed to be heretical is proof that such dissenters existed. Some of those people genuinely were heretics (not that Christians are allowed to burn heretics), others were not.
It is noteworthy that among those considered heretics were those who would dare to translate the word of God into the language of the laity. John Wycliffe's remains were desecrated for translating the Bible into English, and William Tyndale was burned at the stake for attempting the same task. That, in and of itself, should show that the Catholic Church was about as corrupt as a Church could possibly ever become. Few sins are more grievous than suppressing the word of God.
That was not the first time in history God's people were in the minority. A noteworthy example can be found in 1 Kings, in which most of Israel had fallen into idolatry, worshipping a pagan god named Baal. Only 7,000 Jews remained faithful to God. Thus, the mere fact that Catholicism was dominant throughout history does not mean there were no Christians who refused to accept the Catholic religion. Again, the mere fact that they were frequently killed by the Catholic Church shows that they did exist. There were 7,000 Jews who did not bow to Baal, who knows how many Christians refused to bow to the Popes?
So you see that not only does the Catholic Church typically misunderstand the "Protestant" view of the Reformation, the view we do hold is not exactly unthinkable. It wouldn't be the first time in history God's people were in the minority, and the scriptures indicate that it won't be the last. In the mean time, all we can do as Christians is seek refuge in God and know that whether we stand in the majority or in the minority, God will always be true, and we are reserved to Him as those who refused to bow the knee to Baal.