/summa/FS/FSV/FS_Q79:
We must now consider the external causes of sin, and (1) on the part of God; (2) on the part of the devil; (3) on the part of man.
Under the first head there are four points of inquiry:
(1) Whether God is a cause of sin?
(2) Whether the act of sin is from God?
(3) Whether God is the cause of spiritual blindness and hardness of heart?
(4) Whether these things are directed to the salvation of those who are blinded or hardened?