Mark 4:10–12 can create a lot of confusion. The key idea of these verses, however, is that Jesus teaches in parables in order to divide between insiders and outsiders. We often think that parables, being stories, are meant to make things easier to understand. However, the opposite is true. Parables act like a sieve. Those who want to follow Jesus come to him for further explanation and insight (insiders). Those who do not bother to do so will hear but not understand and their indifference will judge them (outsiders).
Knowing the purpose of parables, we are then left with a question: why do some people want to follow Jesus and come to him while others do not? A simple answer to this question is that God in his wisdom has elected some people to eternal life (Ephesians 1:3–14, Ephesians 2:1–10) while leaving others in their sins and misery. That is why some people respond in faith when they hear the gospel, but others do not. The Bible’s teaching regarding God’s sovereignty in salvation is very clearly summarized in the the Canons of Dort, Chapter 1.https://threeforms.org/canons-of-dort/#election
If parables are there to divide between insiders and outsiders, the application for us is not to wonder, Am I an insider?
but rather to determine that we are going to try our best to actively listen to Jesus’ words because that is what he expects from us. We are going to be active rather than passive listeners.
10 And when he was alone, those around him with the twelve asked him about the parables.