In the passage from Jonah, we get Jonah's second chance to answer God's call. We recall that when God gave him his marching orders to go east to Nineveh, Jonah headed west instead. After a close encounter with a big fish, Jonah is given this second chance to go to Nineveh and preach. This time he goes and follows orders, although we know from the end of the story that he was less than happy about this. And here's the contrast. The people of Nineveh don't need a second chance - they repent right away.
In Mark's account of the calling of Andrew, Peter, James, and John, there is no need of a second chance. For whatever reason, these four fisherman down their nets and follow Jesus. Jesus calls them and they follow. In this sense, as in so many others, Jesus is not a typical rabbi who would wait around for potential disciples and would send them off on their own when they were qualified to be rabbis themselves. It is Jesus who calls, and not the disciples who request admission to his rabbinical school. And there is no graduation, no time when we stop being disciples of Jesus.
We might wonder, as Christian have for centuries, what moved these four fisherman to drop everything and become disciples of Jesus. Such wondering is probably beside the point. The point, as Mark tells the story, is that they did. Jesus comes proclaiming that the time has been fulfilled and that the reign of God is near. This is Good News - or one might say - news of the victory of God, for that is how the Greek word was often used, for news of victory in battle. God is victorious, but the announcing of that victory comes with an invitation, a call, first to repentance, to changing one's life direction, and second to believing this Good News. Belief is not simply a matter of intellectual assent to the truth, but of trusting and acting upon that truth. To use an analogy suggested by the idea that the time is fulfilled, a husband's intellectual assent to his pregnant wife's announcement that it's time is not enough, action is required if she is to get to the hospital for the baby's birth.
What is being birthed in us is what was already birthed in Jesus, the reign of God. That reign is present in Jesus and it is perhaps enough to say that Peter, Andrew, James, and John see that and follow. Do we see that in Jesus, and in one another? Do we see in one another lives that are being live towards God and not towards the world's idols? Do we see that in ourselves? Are we willing to let the reign of God be in us as it is in Jesus?