Objection 1. It would seem that hope is not a virtue. For "no man makes ill use of a virtue," as Augustine states (De Lib. Arb. ii, 18). But one may make ill use of hope, since the passion of hope, like the other passions, is subject to a mean and extremes. Therefore hope is not a virtue.
Objection 2. Further, no virtue results from merits, since "God works virtue in us without us," as Augustine states (De Grat. et Lib. Arb. xvii). But hope is caused by grace and merits, according to the Master (Sent. iii, D, 26). Therefore hope is not a virtue.
Objection 3. Further, "virtue is the disposition of a perfect thing" (Phys. vii, text. 17,18). But hope is the disposition of an imperfect thing, of one, namely, that lacks what it hopes to have. Therefore hope is not a virtue.
On the contrary, Gregory says (Moral. i, 33) that the three daughters of Job signify these three virtues, faith, hope and charity. Therefore hope is a virtue.
I answer that, According to the Philosopher (Ethic. ii, 6) "the virtue of a thing is that which makes its subject good, and its work good likewise." Consequently wherever we find a good human act, it must correspond to some human virtue. Now in all things measured and ruled, the good is that which attains its proper rule: thus we say that a coat is good if it neither exceeds nor falls short of its proper measurement. But, as we stated above (8, 3, ad 3) human acts have a twofold measure; one is proximate and homogeneous, viz. the reason, while the other is remote and excelling, viz. God: wherefore every human act is good, which attains reason or God Himself. Now the act of hope, whereof we speak now, attains God. For, as we have already stated (I-II, 40, 1), when we were treating of the passion of hope, the object of hope is a future good, difficult but possible to obtain. Now a thing is possible to us in two ways: first, by ourselves; secondly, by means of others, as stated in Ethic. iii. Wherefore, in so far as we hope for anything as being possible to us by means of the Divine assistance, our hope attains God Himself, on Whose help it leans. It is therefore evident that hope is a virtue, since it causes a human act to be good and to attain its due rule.
Reply to Objection 1. In the passions, the mean of virtue depends on right reason being attained, wherein also consists the essence of virtue. Wherefore in hope too, the good of virtue depends on a man's attaining, by hoping, the due rule, viz. God. Consequently man cannot make ill use of hope which attains God, as neither can he make ill use of moral virtue which attains the reason, because to attain thus is to make good use of virtue. Nevertheless, the hope of which we speak now, is not a passion but a habit of the mind, as we shall show further on (5; 18, 1).
Reply to Objection 2. Hope is said to arise from merits, as regards the thing hoped for, in so far as we hope to obtain happiness by means of grace and merits; or as regards the act of living hope. The habit itself of hope, whereby we hope to obtain happiness, does not flow from our merits, but from grace alone.
Reply to Objection 3. He who hopes is indeed imperfect in relation to that which he hopes to obtain, but has not as yet; yet he is perfect, in so far as he already attains his proper rule, viz. God, on Whose help he leans.
Source: Summa Theologica (New Advent)