Read 2 Corinthians 3.
Why is it that when people hear the gospel or read the Bible, some believe but others are unmoved, unchanged? How can someone study the Bible for years without coming to believe that Jesus is the Christ and that he died on behalf of sinners? How can a devout Jewish person read Isaiah 53 without falling to his knees to confess Christ, calling on God to save him?
The answer to these questions is here in 2 Corinthians 3: 14-16: “But their minds were made dull, for to this day the same veil remains when the old covenant is read. It has not been removed, because only in Christ is it taken away. Even to this day when Moses is read, a veil covers their hearts. But whenever anyone turns to the Lord, the veil is taken away.”
Anyone who knows how to read can read and interpret the Bible properly, but not everyone can believe what the Bible says.
Here in 2 Corinthians 3, Paul tells us that this is because we are blind, by nature, to God’s truth in Christ. The reason why people don’t see him in the Bible and come to trust in him is that “a veil covers their hearts” (v. 15). Only the work of the Spirit of God (v. 17) can welcome Jesus and believe the Bible when they encounter his truth.
Then, once we’ve come to know Christ by faith, we can study the Bible’s depiction of Christ clearly without the veil of unbelief over our eyes (vv. 17-18).
This is how we should pray for unbelievers that we’re witnessing to–that God’s spirit would open their spiritual eyes to see the light of Christ in the gospel. It is also why salvation does not depend on how good your presentation skills are or how perfect your arguments for Christianity may be.
Salvation is a spiritual act; it requires the work of God’s Holy Spirit. So ask for the Spirit’s help when you witness, the Spirit’s illumination, conviction of sin, and regeneration in the hearts of those to whom you witness.