What does Mark 4:12 mean?
The quote Jesus gives is a paraphrase of Isaiah 6:9–10. The context is that Isaiah should continue to preach even though the people are already hardened to his message. Like the sower who spreads seed on the hardened, rocky, and weed-infested soil as well as the fertile ground, Jesus continues to teach the crowds, even knowing they include many who will not accept what He says.This verse begins with a Greek word, hina, which is difficult to translate in this context. Typically translated as "that," or "to," here it most likely means a natural fulfillment: Jesus spoke in parables and the people didn't understand what He was saying. Those who do not allow for a healthy measure of understanding will not perceive or understand and, therefore, will not be forgiven. Or it could refer to God further hardening the hearts of those who hardened their hearts first, like Pharaoh (Exodus 8:15, 32; 9:12). Either way, Jesus' family and some of the priests and Pharisees who later follow Jesus show that this can be a temporary condition. It's also possible that "so that" means this situation occurs "so that" Isaiah's prophecy will be fulfilled.
Matthew 13:14–15 and Isaiah 6:9–10 both say "be healed" instead of "be forgiven." In the Jewish culture, injury and illness are presumed to be punishment for sin (John 9:2), and being healed is public proof that God forgives the victim's sin. This speaks back to Mark 4:11 which explains that even Jesus' actions are in parable form—those who don't care to consider them carefully won't understand the message. The crowds who refuse to dig deeper into His message can't understand that just as it is Jesus who heals them, it is Jesus who can forgive their sins. The Pharisees and casual observers can't understand this even when Jesus spells it out for them (Mark 2:1–12).