St. Alphonsus, in his own time, and those he quotes in their own times, says about this:
“St. Remigius writes that, if children be excepted, the number of adults that are saved is few on account of the sins of the flesh. In conformity with this doctrine, it was revealed to a holy soul that as pride has filled hell with devils, so impurity fills it with men. St. Isidore assigns the reason. He says that there is no vice which so much enslaves men to the devil as impurity. Hence St. Augustine says that with regard to this sin, “the combat is common and the victory rare.” Hence it is that on account of this sin hell is filled with souls.” (Sermons for all the Sundays in the Year, Sermon 45, On Impurity)
We already know what Our Lord says in the Bible. Our Lady of Fatime also is obvious.
St. Leonard of Port Maurice (A.D. 1676-1751), speaks on the fewness of the saved: “After consulting all the theologians and making a diligent study of the matter, he [Suarez] wrote, ‘The most common sentiment which is held is that, among Christians [Catholics], there are more damned souls than predestined souls.’ Add the authority of the Greek and Latin Fathers to that of the theologians, and you will find that almost all of them say the same thing. This is the sentiment of Saint Theodore, Saint Basil, Saint Ephrem, Saint John Chrysostom. What is more, according to Baronius it was a common opinion among the Greek Fathers that this truth was expressly revealed to Saint Simeon Stylites and that after this revelation, it was to secure his salvation that he decided to live standing on top of a pillar for forty years, exposed to the weather, a model of penance and holiness for everyone. Now let us consult the Latin Fathers. You will hear Saint Gregory saying clearly, ‘Many attain to faith, but few to the heavenly kingdom.’ Saint Anselm declares, ‘There are few who are saved.’ Saint Augustine states even more clearly, ‘Therefore, few are saved in comparison to those who are damned.’ The most terrifying, however, is Saint Jerome. At the end of his life, in the presence of his disciples, he spoke these dreadful words: ‘Out of one hundred thousand people whose lives have always been bad, you will find barely one who is worthy of indulgence.’ (On The Little Number of Those Who Are Saved, by St. Leonard of Port Maurice)