Objection 1. It would seem that Christ's predestination is not the cause of ours. For that which is eternal has no cause. But our predestination is eternal. Therefore Christ's predestination is not the cause of ours.
Objection 2. Further, that which depends on the simple will of God has no other cause but God's will. Now, our predestination depends on the simple will of God, for it is written (Ephesians 1:11): "Being predestinated according to the purpose of Him, Who worketh all things according to the counsel of His will." Therefore Christ's predestination is not the cause of ours.
Objection 3. Further, if the cause be taken away, the effect is also taken away. But if we take away Christ's predestination, ours is not taken away; since even if the Son of God were not incarnate, our salvation might yet have been achieved in a different manner, as Augustine says (De Trin. xiii, 10). Therefore Christ's predestination is. not the cause of ours.
On the contrary, It is written (Ephesians 1:5): "(Who) hath predestinated us unto the adoption of children through Jesus Christ."
I answer that, if we consider predestination on the part of the very act of predestinating, then Christ's predestination is not the cause of ours; because by one and the same act God predestinated both Christ and us. But if we consider predestination on the part of its term, thus Christ's predestination is the cause of ours: for God, by predestinating from eternity, so decreed our salvation, that it should be achieved through Jesus Christ. For eternal predestination covers not only that which is to be accomplished in time, but also the mode and order in which it is to be accomplished in time.
Replies to Objections 1 and 2. These arguments consider predestination on the part of the act of predestinating.
Reply to Objection 3. If Christ were not to have been incarnate, God would have decreed men's salvation by other means. But since He decreed the Incarnation of Christ, He decreed at the same time that He should be the cause of our salvation.
Source: Summa Theologica (New Advent)