O LORD, why have you made us to err from your ways, and hardened our heart from your fear? Return for your servants' sake, the tribes of your inheritance.
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God does not bestow on some the help for avoiding sin which He bestows on others. This help is not merely the infusion of grace, but also an exterior guardianship, whereby the occasions of sin are providentially removed from a man’s path. God also aids man against sin by the natural light of reason, and other natural goods that He bestows on man. When then He withdraws these aids from some, as their conduct deserves that he should, according to the exigency of His justice, He is said to harden them, or to blind them. (St. Thomas Aquinas On God and His Creaure 3.163.4)