Objection 1. It would seem that we ought to observe some mode in loving God. For the notion of good consists in mode, species and order, as Augustine states (De Nat. Boni iii, iv). Now the love of God is the best thing in man, according to Colossians 3:14: "Above all... things, have charity." Therefore there ought to be a mode of the love of God.
Objection 2. Further, Augustine says (De Morib. Eccl. viii): "Prithee, tell me which is the mode of love. For I fear lest I burn with the desire and love of my Lord, more or less than I ought." But it would be useless to seek the mode of the Divine love, unless there were one. Therefore there is a mode of the love of God.
Objection 3. Further, as Augustine says (Gen. ad lit. iv, 3), "the measure which nature appoints to a thing, is its mode." Now the measure of the human will, as also of external action, is the reason. Therefore just as it is necessary for the reason to appoint a mode to the exterior effect of charity, according to Romans 12:1: "Your reasonable service," so also the interior love of God requires a mode.
On the contrary, Bernard says (De Dilig. Deum 1) that "God is the cause of our loving God; the measure is to love Him without measure."
I answer that, As appears from the words of Augustine quoted above (Objection 3) mode signifies a determination of measure; which determination is to be found both in the measure and in the thing measured, but not in the same way. For it is found in the measure essentially, because a measure is of itself the determining and modifying rule of other things; whereas in the things measured, it is found relatively, that is in so far as they attain to the measure. Hence there can be nothing unmodified in the measure whereas the thing measured is unmodified if it fails to attain to the measure, whether by deficiency or by excess.
Now in all matters of appetite and action the measure is the end, because the proper reason for all that we desire or do should be taken from the end, as the Philosopher proves (Phys. ii, 9). Therefore the end has a mode by itself, while the means take their mode from being proportionate to the end. Hence, according to the Philosopher (Polit. i, 3), "in every art, the desire for the end is endless and unlimited," whereas there is a limit to the means: thus the physician does not put limits to health, but makes it as perfect as he possibly can; but he puts a limit to medicine, for he does not give as much medicine as he can, but according as health demands so that if he give too much or too little, the medicine would be immoderate.
Again, the end of all human actions and affections is the love of God, whereby principally we attain to our last end, as stated above (Question 23, Article 6), wherefore the mode in the love of God, must not be taken as in a thing measured where we find too much or too little, but as in the measure itself, where there cannot be excess, and where the more the rule is attained the better it is, so that the more we love God the better our love is.
Reply to Objection 1. That which is so by its essence takes precedence of that which is so through another, wherefore the goodness of the measure which has the mode essentially, takes precedence of the goodness of the thing measured, which has its mode through something else; and so too, charity, which has a mode as a measure has, stands before the other virtues, which have a mode through being measured.
Reply to Objection 2. As Augustine adds in the same passage, "the measure of our love for God is to love Him with our whole heart," that is to love Him as much as He can be loved, and this belongs to the mode which is proper to the measure.
Reply to Objection 3. An affection, whose object is subject to reason's judgment, should be measured by reason. But the object of the Divine love which is God surpasses the judgment of reason, wherefore it is not measured by reason but transcends it. Nor is there parity between the interior act and external acts of charity. For the interior act of charity has the character of an end, since man's ultimate good consists in his soul cleaving to God, according to Psalm 72:28: "It is good for me to adhere to my God"; whereas the exterior acts are as means to the end, and so have to be measured both according to charity and according to reason.
Source: Summa Theologica (New Advent)