From Origen – Commentary on the Epistle to the Romans:

(4) After all, later in the letter he himself explains this more fully when he says, “For those whom he foreknew he also predestined to be conformed to the image of his Son.”46 Plainly showing that those whom God foreknew would become the kind to conform themselves to Christ by their sufferings, he even predestined them to be conformed and similar to his image and glory. Therefore there precedes a foreknowledge of them, through which is known what effort and virtue they will understand at once that they are set apart from the womb deservedly.
(3) It says then that Paul was set apart for the gospel and set apart from his own mother’s womb. The reasons for this and the merits which entitled him to be set apart for this purpose were seen by the One from whom man’s mind does not escape. For God foresaw that Paul was going to labor harder than all the others in the gospel; that, despite hunger and thirst, cold and nakedness, dangers from thieves, dangers from rivers, dangers at sea, he was going to preach the gospel of Christ, knowing that it would have been woe to him if he did not preach the gospel; and that he was going to punish his body and reduce it to slavery, so that, after proclaiming to others, he himself should not be rejected. Therefore, seeing in advance these things and many other similar things in him,God set Paul apart for the gospel from his mother’s womb on account of these matters. For if, as the heretics think, he had been chosen either by uncertain fate or by the privilege of possessing a superior nature, surely he would never have expressed the fear that, if he were not to hold the restraints on his own body, it could potentially come to pass that he would be rejected or that woe would be his if he were to cease from proclaiming the gospel.
(4) After all, later in the letter he himself explains this more fully when he says, “For those whom he foreknew he also predestined to be conformed to the image of his Son.” Plainly showing that those whom God foreknew would become the kind to conform themselves to Christ by their sufferings, he even predestined them to be conformed and similar to his image and glory. Therefore there precedes a foreknowledge of them, through which is known what effort and virtue they will possess in themselves, and thus predestination follows, yet foreknowledge should not be considered the cause of predestination. For while men requite merit to each individual based upon past accomplishments, for God this is determined from future ones; and a person is very impious not to concede to God that what we see in the past he can see in the future.