In the widest sense it is every eternal decision of God; in a narrower sense it is the supernatural final destination of rational creatures; and in the strictest sense it is God's eternal decision to assume certain rational creatures into heavenly glory. Predestination implies an act of the divine intellect and of the divine will. The first is foreknowledge, the second is predestination.
According to its efficacy in time, predestination is distinguished as incomplete or complete depending on whether it is to grace only or also to glory. Complete predestination is the divine preparation of grace in the present life and of glory in the life to come.
This doctrine is proposed by the ordinary and universal teaching of the Church as a truth of revelation. The reality of predestination is clearly attested by St. Paul: "They are the ones he chose especially long ago and intended to become true images of the Son, so that his Son, might be the eldest of many brothers. He called those he intended for this; those he called he justified and with those he justified he shared his glory." (Romans 8:29-30). All elements of complete predestination are given: the activity of God's mind and will, and the principal stages of its realization in time.
The main difficulty in the doctrine of predestination is whether God's eternal decision has been taken with or without consideration of human freedom. Catholic teaching holds that predestination by God does not deny the human free will. Numerous theories have been offered on how to reconcile the two, but all admit with St. Paul (Romans 11:33) that predestination is an unfathomable mystery. (Etym. Latin praedestinatio, a determining beforehand.)