"Catholics love the Sermon on the Mount (Matthew ch5-7) but Protestants generally avoid it, since it doesn't fit with their ideas of how salvation is supposed to take place."
Before we begin this critique, the question of why the author would make such a claim needs to be answered. He cites Matthew 7:13-14 as his proof text, and then asks a series of rhetorical questions:
"But given the above teaching of Jesus, what is so "difficult" about the Faith Alone approach? What is so "narrow" path about it? Why are "few" saved if all they need to do is believe?"
The Sermon on the Mount is about adhering to the spirit of the Law, not just the letter. God does not merely examine behavior, but the heart. People practice various sins because their hearts are already dedicated to unrighteousness (Matthew 5:21-28). Our deeds are the evidence of what is taking place in our hearts. Jesus obviously had the self-righteous, hypocritical Scribes and Pharisees in mind when He gave this sobering speech (Matthew 5:20). He is providing a description of what His true disciples will be like. The "narrow path" is "difficult" because it flies right in the face of everything that our sinful nature prompts us to do. Justification before God can only be found in Christ. He is the only One who can save us. Everybody who asks can receive eternal life (Matthew 7:7-8). Trusting in Christ is "easy" (Matthew 11:28-29). The moment of conversion is not to be conflated with following Jesus on a daily basis.
"Protestants typically 'interpret' the teachings of Jesus as being either (1) meant for Old Testament folks alone, or (2) simply to show us how sinful we are, not to actually impose any commands or expectations upon us. Such is quite absurd, and effectively renders the Gospels hollow."
Surely, we can all benefit considerably from studying the Sermon on the Mount. It gives us a clear picture of how God wants us to be. Our good works are the product of a changed heart. Our principle care and focus in this life should be on getting to heaven. The only way for us to get saved is by trusting in the work of the Lord Jesus Christ. Our chief concern in this life should be on proclaiming the gospel. We should be continually pursuing after God's righteousness. No person who has a biblically solid worldview would ever claim that following Jesus is optional. It is not as though Jesus here is prescribing sacraments for salvation or some monastic vows. In short, the Sermon on the Mount does absolutely nothing to refute Sola Fide.