How do we know that God must open our hearts to believe?

In Acts 16:14, the phrase 'whose heart the Lord opened' underscores the necessity of divine intervention in the human heart for genuine faith to occur. Without God's grace to open our hearts, we remain spiritually blind and deaf to the saving message of Christ. This parallels with the biblical assertion that the natural man does not receive the things of the Spirit (1 Corinthians 2:14).

Furthermore, the transformation seen in Lydia—her worship, attentiveness to Paul's preaching, and subsequent baptism—all flow from the initial act of God opening her heart. This affirms the Reformed perspective that regeneration precedes faith; only when the Holy Spirit transforms an individual can they recognize and embrace the truth of the gospel. In essence, faith is a gift from God, revealed to us by His grace.