Is the famous sermon of St. Leonard of Port Maurice on the fewness of the saved reliable?

I have not read or heard that particular sermon, but the idea that few are saved is not an uncommon understanding of the Bible from cover to cover. Six chapters into Scripture, out of its 1,189 chapters, God is already destroying the world for its wickedness, sparing only eight people. That tells you right away this is not going to be a happy story. We just went through the history of the Israelites and their faithlessness, where God wanted to destroy them. Story after story tells us few will be saved. When Elijah in Romans 11 is ready to give up because no one listens, God's encouragement is that there are seven thousand who have not bowed the knee to the false god, out of a nation of perhaps five to ten million. Elijah does not object that seven thousand is few, because God is not looking for big numbers; He is looking for faithful people. In Jeremiah 5:1 God says that if He can find even one person who does what is right, He will spare the whole city, echoing Abraham bargaining down to ten righteous men for Sodom. So when Jesus says many are called but few are chosen, or take the narrow gate because wide is the way of destruction, He is following a pattern already built into Scripture. God does not exaggerate or sugarcoat it; He is telling the truth, and on Judgment Day there will be much weeping and wailing. Be glad you are listening, because you can still do something about it.