The church fathers particularly emphasize the theme of patience when interpreting the parable of the wheat and tares. Several reasons are given for this patience, the most prominent reason being "time for repentance." Among the several fathers who stressed this theme, I found the most striking--and comforting--in these words of St Peter Chrysologus:
If the patience of God did not come to the aid of the tares, the Church would not have either Matthew as an evangelist after having been a tax collector, or Paul as an apostle after having been a persecutor.
And so Ananias was seeking to uproot the wheat on that occasion when he was sent to Saul and made this complaint about Paul: "Lord, how great are the evils which he did against your saints!" What he means is, "Uproot the tares! Why send a sheep to a wolf? Why send a devoted servant to the insolent? Why send such a preacher to a persecutor?" But Ananias had seen Saul, while the Lord was seeing Paul. When Ananias was calling him a persecutor, the Lord already knew him as a preacher; and when he was judging him to be tares destined for hell, Christ already had a hold on [Paul] as a chosen vessel, wheat for the heavenly granary. Do you not know, he says, "that he is a chosen vessel of mine?"
Source: The Fathers of the Church: St. Peter Chrysologus, Selected Sermons (110.104)