Objection 1. It would seem that theological virtue observes the mean. For the good of other virtues consists in their observing the mean. Now the theological virtues surpass the others in goodness. Therefore much more does theological virtue observe the mean.
Objection 2. Further, the mean of moral virtue depends on the appetite being ruled by reason; while the mean of intellectual virtue consists in the intellect being measured by things. Now theological virtue perfects both intellect and appetite, as stated above (Question 62, Article 3). Therefore theological virtue also observes the mean.
Objection 3. Further, hope, which is a theological virtue, is a mean between despair and presumption. Likewise faith holds a middle course between contrary heresies, as Boethius states (De Duab. Natur. vii): thus, by confessing one Person and two natures in Christ, we observe the mean between the heresy of Nestorius, who maintained the existence of two persons and two natures, and the heresy of Eutyches, who held to one person and one nature. Therefore theological virtue observes the mean.
On the contrary, Wherever virtue observes the mean it is possible to sin by excess as well as by deficiency. But there is no sinning by excess against God, Who is the object of theological virtue: for it is written (Sirach 43:33): "Blessing the Lord, exalt Him as much as you can: for He is above all praise." Therefore theological virtue does not observe the mean.
I answer that, As stated above (Article 1), the mean of virtue depends on conformity with virtue's rule or measure, in so far as one may exceed or fall short of that rule. Now the measure of theological virtue may be twofold. One is taken from the very nature of virtue, and thus the measure and rule of theological virtue is God Himself: because our faith is ruled according to Divine truth; charity, according to His goodness; hope, according to the immensity of His omnipotence and loving kindness. This measure surpasses all human power: so that never can we love God as much as He ought to be loved, nor believe and hope in Him as much as we should. Much less therefore can there be excess in such things. Accordingly the good of such virtues does not consist in a mean, but increases the more we approach to the summit.
The other rule or measure of theological virtue is by comparison with us: for although we cannot be borne towards God as much as we ought, yet we should approach to Him by believing, hoping and loving, according to the measure of our condition. Consequently it is possible to find a mean and extremes in theological virtue, accidentally and in reference to us.
Reply to Objection 1. The good of intellectual and moral virtues consists in a mean of reason by conformity with a measure that may be exceeded: whereas this is not so in the case of theological virtue, considered in itself, as stated above.
Reply to Objection 2. Moral and intellectual virtues perfect our intellect and appetite in relation to a created measure and rule; whereas the theological virtues perfect them in relation to an uncreated rule and measure. Wherefore the comparison fails.
Reply to Objection 3. Hope observes the mean between presumption and despair, in relation to us, in so far, to wit, as a man is said to be presumptuous, through hoping to receive from God a good in excess of his condition; or to despair through failing to hope for that which according to his condition he might hope for. But there can be no excess of hope in comparison with God, Whose goodness is infinite. In like manner faith holds a middle course between contrary heresies, not by comparison with its object, which is God, in Whom we cannot believe too much; but in so far as human opinion itself takes a middle position between contrary opinions, as was explained above.
Source: Summa Theologica (New Advent)