Since faith is infinitely beyond all the power of our unregenerated human nature, it is only God who can give the spiritual ears to hear and eyes to see the beauty of Christ in the gospel. God alone disarms the hostility of the sinner turning his heart of stone to a heart of flesh. It is God, the Holy Spirit, alone who gives illumination and understanding of His word that we might believe; It is God who raises us from the death of sin, who circumcises the heart; unplugs our ears; It is God alone who can give us a new sense, a spiritual capacity to behold the beauty and unsurpassed excellency of Jesus Christ.
The apostle John recorded Jesus saying to Nicodemus that we naturally love darkness, hate the light and WILL NOT come into the light (John 3:19, 20). And since our hardened resistance to God is thus seated in our affections, only God, by His grace, can lovingly change, overcome and pacify our rebellious disposition. The natural man, apart from the quickening work of the Holy Spirit, will not come to Christ on his own since he is at enmity with God and cannot understand spiritual things (1 Cor 2:14). Shining a light into a blind man’s eyes will not enable him to see, because eyesight first requires a set of healthy eyes. Likewise, reading or hearing the word of God alone cannot elicit saving faith in the reader (1 Thess 1:4, 5) unless God plows up the fallow ground of our hearts and the Spirit “germinates” the seed of the word, opening our eyes to see Christ’s true beauty and excellency and uniting us to Him through a Spirit-wrought faith.
So the problem of conversion is not with the Word or God’s Law but with man’s prideful heart. The humility required to submit to the gospel is, therefore, not prompted by man’s will but by God’s mercy (Rom 9:16) since no one can believe the gospel unless God grants it (John 6:63, 65). As an example of how the Spirit uses the means of the spoken word to disarm closed hearts, the Book of Acts records the work of the Holy Spirit during the preaching of the apostles and, in one instance, states that “the Lord opened her [Lydia’s] heart to respond to the things spoken by Paul,” (Acts 16:14). The Spirit must likewise give all His people spiritual life and understanding if their hearts are to be opened and thus respond to Christ in faith.
Via: Reformation Theology