This gospel has two distinct parts to it. It has the first part about Jesus preaching about the kingdom of God, and the second part where Jesus calls his first disciples. Even if we take the two parts separately, there are powerful messages in each of them for us. But if we take them together, we encounter a call we can't avoid.
"There must have been something divinely compelling in the face of the Savior, Otherwise they would not have acted so irrationally as to follow a man whom they had never seen before. Does one leave a father to follow a man in whom he sees nothing more than he sees in his father? They left their father of the flesh to follow the Father of the spirit. They did not leave a father; they found a Father. What is the point of this digression? To show that there was something divine in the Savior's very countenance that men, seeing, could not resist."
St. Jerome asks a good question: "Does one leave a father to follow a man in whom he sees nothing more than he sees in his father?" No! You don't! So there must have been something truly compelling in the person of Jesus. Not that there was anything wrong with their own fathers and their old lives, but we have to deal with the facts of the story. Something about Jesus made these men, and men and women over the next 2000 years, turn their backs on all the good things of their lives in pursue him. Back in high school when I was discerning seminary, I was pointed to this gospel passage on several occasions to pray with the radical surrender that these men showed when Jesus called them. In seeing how Peter, Andrew, James, and John left everything to follow Jesus, it helped to give me courage to try to respond to Jesus' invitation the same way. And what did I find? That just like Jesus never let the apostles down once they decided to follow him, he has never let me down either.
Repenting and believing go hand in hand. Believing something with your head or your heart means very little if it's not reflected in your life, so repentance is crucial. But repentance doesn't just mean to stop doing bad things. The Greek word for repent, μετανοέω, implies a new or more complete understanding. Μετά means after, and νοέω means to understand. So the Greek word for repent means an "after-understanding" or a more complete understanding. Sin points to a misunderstanding about the love of God, so repentance is not just giving up your sins and getting rid of them, but repentance means replacing your sins with a more complete way of understanding, so after you give up the sin you understand God in a new way.
And this makes sense if we think about a small example like bad habits. Whatever your bad habit might be, smoking, swearing, or whatever, you usually can't give it up by just stopping. You have to replace it with something else. Similarly, Christian repentance, the repentance that Jesus commands us to do today, is not just giving up sin and vice, but replacing them with the virtues that draw you closer to God. So if your sin is hatred, it's not enough to just stop hating, because that's not how the human spirit works. You have to develop the opposing virtue of patience. If your sin is sloth or laziness, you can't just say you're going to stop being lazy, you have to actually work at the virtue of diligence. When it comes to vice and virtue, sin and repentance, there is no middle ground. When it comes to Jesus' command to repent and believe in the gospel, there is no middle ground.
And then, once you ask him, don't settle for a vague answer from him or a vague plan from yourself. If you want to follow Jesus closer it takes concrete steps. So don't settle for, "I need to be nicer to others" because how do you measure that? Rather, your repentance should take the form of, "I need to forgive this specific person." Don't settle for, "I need to pray more" because again, how do you measure that? Rather, your repentance, your plan for following Jesus, should be like, "I'm going to pray fifteen minutes a day, at the very beginning of the day before it gets busy."
This call to repentance, this call from Jesus to follow him, it's urgent my friends, because the kingdom of God is at hand. This kingdom is coming when Christ comes again in glory, and it's already in our midst, through the Church and through the Sacraments. So the call to follow Jesus is not something we can put off until tomorrow. Take the apostles as your example. When Jesus called them, they didn't know what the end was going to be, they didn't even know what the next day was going to hold. They just knew the next step. They knew they couldn't stay where they were at, they knew they had to drop their nets, stand up, and start walking. Just the same for you, you don't have to know the end, you don't have to know where Jesus is taking you years from now. You just need to ask him what that next step is, and then do it. Don't settle for mediocrity, don't settle for "I'm ok as I am," but constantly strive to follow him closer. Repent, and believe in the gospel.