We ought to pray even for sinners, that they may be converted, and for the just that they may persevere and advance in holiness. Yet those who pray are heard not for all sinners but for some: since they are heard for the predestined, but not for those who are foreknown to death; even as the correction whereby we correct the brethren, has an effect in the predestined but not in the reprobate, according to Ecclesiastes 7:14, “No man can correct whom God hath despised.” Hence it is written (1 John 5:16): “He that knoweth his brother to sin a sin which is not to death, let him ask, and life shall be given to him, who sinneth not to death.” Now just as the benefit of correction must not be refused to any man so long as he lives here below, because we cannot distinguish the predestined from the reprobate, as Augustine says (De Correp. et Grat. xv), so too no man should be denied the help of prayer.
Thomas Aquinas, Summa Theologica (II-II Question 83, Article 7, Reply to Objection 3)
