The cause or blame for this unbelief, as well as for all other sins, is not at all in God, but in man. Faith in Jesus Christ, however, and salvation through him is a free gift of God. As Scripture says, It is by grace you have been saved, through faith, and this not from yourselves; it is a gift of God (Eph. 2:8). Likewise: It has been freely given to you to believe in Christ (Phil. 1:29).
The fact that some receive from God the gift of faith within time, and that others do not, stems from his eternal decision. For all his works are known to God from eternity (Acts 15:18; Eph. 1:11). In accordance with this decision he graciously softens the hearts, however hard, of his chosen ones and inclines them to believe, but by his just judgment he leaves in their wickedness and hardness of heart those who have not been chosen. And in this especially is disclosed to us his act — unfathomable, and as merciful as it is just — of distinguishing between people equally lost. This is the well-known decision of election and reprobation revealed in God’s Word. This decision the wicked, impure, and unstable distort to their own ruin, but it provides holy and godly souls with comfort beyond words.