In Matthew 4:17 the Lord’s preaching is summarized by St. Matthew in this way: Repent, for the kingdom of heaven is at hand. I say that Matthew summarized the Lord’s preaching thusly because we do not find many occasions in the Gospels where Jesus is reported to have actually used this expression. That’s not to say that He never did, of course, but because we have examples of all sorts of sermons and messages that He preached, it seems fair to conclude that Matthew’s point is that Repent, for the kingdom of heaven is at hand is the essential point of what the Lord Jesus proclaimed.
I think that this is an important observation to make because it gives us a rule of thumb for understanding Jesus’s teaching. If we find ourselves confused by something that He said, will any light be shed upon it by holding it up against Matthew’s summary? I think that it would, and I do not think this is an unreasonable expectation so long as we do not make a Procrustean bed out the thing, forcing literally everything said by the Lord to fit just so. One reason that I think this is important is that it I believe that this verse fundamentally undermines the Protestant notion of sola fide. Here’s why.
To repent of something is to do something different from exercising faith. To exercise faith, in the Protestant view, is to trust in Christ alone for salvation: Jesus plus nothing. But that is definitely not what Jesus says here. In fact, faith is nowhere to be seen in Matthew 4:17, really. Now it would of course be going rather too far to conclude from this that faith is not necessary for our salvation. It is necessary. But it would be going beyond St. Matthew’s summary of the Lord’s preaching to say that sola fide is what Jesus meant. There are perfectly useful words that express the the idea of faith alone, and Jesus does not use them. He does not say, “Trust in Me alone, for the kingdom of heaven is at hand;” He does not say, “Have faith (alone);” He does not say “All you have to do is believe in Me” either. No. What He said is repent.
So what does repentance look like? Well, whether one is Protestant or Catholic there’s just no denying that repentance means quite a bit more than merely having faith. It includes being sorry for our sins, in such a way that we start working to change how we live our lives. Of course, Catholics say that it means other things as well but in general Protestants would at least agree that merely being sorry for what you’ve done is not sufficient. A change of life is in view, such that we begin to pursue holiness and to love God and our neighbor.
This repentance is that to which Jesus calls us on the grounds that the Kingdom of heaven is at hand. He does not tell us to have faith alone. He tells us to change our lives. We cannot do this, of course, apart from the grace of God. But repentance is fundamental to Jesus’s message. Lord, help us to live lives of repentance such as please You, because we love You and want to be like You.
[Update, 19 March 2016]
I have said elsewhere that I am no expert. And this little addendum just shows to go you, because if I had any sort of scholarly bent at all I might have sought this out when I first published this post. Oh well. 😉
I said above that repentance is distinct from faith. Don’t take my word for it. The Gospel of St. Mark says this in chapter one:
Jesus came to Galilee proclaiming the gospel of God: “This is the time of fulfillment. The kingdom of God is at hand. Repent, and believe in the gospel.” (verses 14-15; emphasis added)
[Update, 23 March 2016]
John the Baptist’s preaching is consistent with the notion that true repentance is more than merely being sorry for my sins.
Produce good fruits as evidence of your repentance. (Luke 3:8)
And:
Every tree that does not produce good fruit will be cut down and thrown into the fire (Luke 3:9).
The life of the truly repentant man is one marked by the pursuit of holiness “without which no one will see the Lord.” So we see that more than mere faith is in view in Gospel preaching.
[Update, Easter 2016]
I tell you, if you do not repent, you will all perish as they did! (Luke 13:5)
Jesus does not say, “If you do not believe, you will all perish”. He repeats this message of repentance twice here (also see Luke 13:3). He doesn’t mention faith here. Of course that does not mean that faith is unnecessary. But the fact that He does insist upon repentance must surely mean something: it means that faith alone is not enough. We must also repent.
Aquinas, Etc. 
Would it be fair to say that the existential American political arena requires the Procrustean method in order to accomplish the summum bonum?
I think it’s fair to say that no American bureaucrat ever saw a Procrustean bed that he didn’t like. I also think it’s fair to say that it is certainly not required. :-)
The medieval rack was a quite reliable tool
in changing one’s attitude….especially the individual vote.
If the electorate was placed upon the rack, would the outcome produce any difference?
No pun intended.
Y’all lost me on this bit about the rack, so I’ll just sit here and smile :-)