In the Old Testament, “ears to hear” was a phrase used of those who could not hear because they were spiritually dead. While it was the fault of the “deaf” individual, God was the one withholding the ability to rightly understand what He commanded because of their sin. It was often used to describe the people of Israel, and as a result of their idolatry.
In the New Testament, Jesus is dealing with different rebellious Jews, the Jewish leaders, who had set themselves as roadblocks to Jesus’ ministry. While He gave them time to repent, when it was clear that they were persistently rebellious, He switched to parables to obscure the truth of salvation from them. This was a judgement against them to prevent them from being saved (Matthew 13:14–15). However, despite the rebellion of the Jewish leadership, God was saving many regular Jews. When Jesus said, “he who has ears” he was talking about those whom God had spiritually awakened so they could understand Jesus’ teaching.
The Word of God is patently clear, particularly when it comes to truths about who God is, who Jesus is, and how we are to be saved. This is referred to as the doctrine of “perspicuity” (clarity). However, despite its clarity, many do not understand it.
That is because the Scriptures require spiritual insight to understand, and those without the Holy Spirit—unbelievers—are unable to fully understand it (1 Corinthians 2:14). Most people do not have ears to hear. It is not because they need more evidence but because God needs to graciously open their ears. Or, as Jesus put it, no one can come to Him unless the Father first draws them (John 6:44).
Prior to salvation, everyone is dead and unable to respond (Ephesians 2:1), “But God, being rich in mercy” makes people alive. He gives men and women the ability to see and hear and understand. While we cannot make people understand, we have a role to play—we are to bring the Gospel to the ends of the earth (Matthew 28:19–20). We are to do our best to make the gospel as clear and as understandable as possible. However, after we do that, we are to then leave salvation to God. We know that, when He desires, He will “open their ears” and bring people from darkness into light (Acts 26:18), and then they won’t be able to help but hear and respond correctly to spiritual things!