Objection 1. It would seem that the angels can change the will of man. For, upon the text, "Who maketh His angels spirits and His ministers a flame of fire" (Hebrews 1:7), the gloss notes that "they are fire, as being spiritually fervent, and as burning away our vices." This could not be, however, unless they changed the will. Therefore the angels can change the will.
Objection 2. Further, Bede says (Super Matth. xv, 11), that, "the devil does not send wicked thoughts, but kindles them." Damascene, however, says that he also sends them; for he remarks that "every malicious act and unclean passion is contrived by the demons and put into men" (De Fide Orth. ii, 4); in like manner also the good angels introduce and kindle good thoughts. But this could only be if they changed the will. Therefore the will is changed by them.
Objection 3. Further, the angel, as above explained, enlightens the human intellect by means of the phantasms. But as the imagination which serves the intellect can be changed by an angel, so can the sensitive appetite which serves the will, because it also is a faculty using a corporeal organ. Therefore as the angel enlightens the mind, so can he change the will.
On the contrary, To change the will belongs to God alone, according to Proverbs 21:1: "The heart of the king is in the hand of the Lord, whithersoever He will He shall turn it."
I answer that, The will can be changed in two ways.
First, from within; in which way, since the movement of the will is nothing but the inclination of the will to the thing willed, God alone can thus change the will, because He gives the power of such an inclination to the intellectual nature. For as the natural inclination is from God alone Who gives the nature, so the inclination of the will is from God alone, Who causes the will.
Secondly, the will is moved from without. As regards an angel, this can be only in one way--by the good apprehended by the intellect. Hence in as far as anyone may be the cause why anything be apprehended as an appetible good, so far does he move the will. In this way also God alone can move the will efficaciously; but an angel and man move the will by way of persuasion, as above explained (106, 2).
In addition to this mode the human will can be moved from without in another way; namely, by the passion residing in the sensitive appetite: thus by concupiscence or anger the will is inclined to will something. In this manner the angels, as being able to rouse these passions, can move the will, not however by necessity, for the will ever remains free to consent to, or to resist, the passion.
Reply to Objection 1. Those who act as God's ministers, either men or angels, are said to burn away vices, and to incite to virtue by way of persuasion.
Reply to Objection 2. The demon cannot put thoughts in our minds by causing them from within, since the act of the cogitative faculty is subject to the will; nevertheless the devil is called the kindler of thoughts, inasmuch as he incites to thought, by the desire of the things thought of, by way of persuasion, or by rousing the passions. Damascene calls this kindling "a putting in" because such a work is accomplished within. But good thoughts are attributed to a higher principle, namely, God, though they may be procured by the ministry of the angels.
Reply to Objection 3. The human intellect in its present state can understand only by turning to the phantasms; but the human will can will something following the judgment of reason rather than the passion of the sensitive appetite. Hence the comparison does not hold.
Source: Summa Theologica (New Advent)