Objection 1. It would seem that an aureole is not due to doctors. For every reward to be had in the life to come will correspond to some act of virtue. But preaching or teaching is not the act of a virtue. Therefore an aureole is not due to teaching or preaching.
Objection 2. Further, teaching and preaching are the result of studying and being taught. Now the things that are rewarded in the future life are not acquired by a man's study, since we merit not by our natural and acquired gifts. Therefore no aureole will be merited in the future life for teaching and preaching.
Objection 3. Further, exaltation in the life to come corresponds to humiliation in the present life, because "he that humbleth himself shall be exalted" (Matthew 23:12). But there is no humiliation in teaching and preaching, in fact they are occasions of pride; for a gloss on Matthew 4:5, "Then the devil took Him up," says that "the devil deceives many who are puffed up with the honor of the master's chair." Therefore it would seem that an aureole is not due to preaching and teaching.
On the contrary, A gloss on Ephesians 1:18-19, "That you may know... what is the exceeding greatness," etc. says: "The holy doctors will have an increase of glory above that which all have in common." Therefore, etc.
Further, a gloss on Canticle of Canticles 8:12, "My vineyard is before me," says: "He describes the peculiar reward which He has prepared for His doctors." Therefore doctors will have a peculiar reward: and we call this an aureole.
I answer that, Just as by virginity and martyrdom a person wins a most perfect victory over the flesh and the world, so is a most perfect victory gained over the devil, when a person not only refuses to yield to the devil's assaults, but also drives him out, not from himself alone, but from others also. Now this is done by preaching and teaching: wherefore an aureole is due to preaching and teaching, even as to virginity and martyrdom. Nor can we admit, as some affirm, that it is due to prelates only, who are competent to preach and teach by virtue of their office. but it is due to all whosoever exercise this act lawfully. Nor is it due to prelates, although they have the office of preaching, unless they actually preach, since a crown is due not to the habit, but to the actual strife, according to 2 Timothy 2:5, "He... shall not be [Vulgate: 'is not'] crowned, except he strive lawfully."
Reply to Objection 1. Preaching and teaching are acts of a virtue, namely mercy, wherefore they are reckoned among the spiritual alms deeds [Cf. II-II, 32, 2].
Reply to Objection 2. Although ability to preach and teach is sometimes the outcome of study, the practice of teaching comes from the will, which is informed with charity infused by God: and thus its act can be meritorious.
Reply to Objection 3. Exaltation in this life does not lessen the reward of the other life, except for him who seeks his own glory from that exaltation: whereas he who turns that exaltation to the profit of others acquires thereby a reward for himself. Still, when it is stated that an aureole is due to teaching, this is to be understood of the teaching of things pertaining to salvation, by which teaching the devil is expelled from men's hearts, as by a kind of spiritual weapon, of which it is said (2 Corinthians 10:4): "The weapons of our warfare are not carnal but spiritual" [Vulgate: 'but mighty to God'].
Source: Summa Theologica (New Advent)