Objection 1. It would seem that God cannot be loved wholly. For love follows knowledge. Now God cannot be wholly known by us, since this would imply comprehension of Him. Therefore He cannot be wholly loved by us.
Objection 2. Further, love is a kind of union, as Dionysius shows (Div. Nom. iv). But the heart of man cannot be wholly united to God, because "God is greater than our heart" (1 John 3:20). Therefore God cannot be loved wholly.
Objection 3. Further, God loves Himself wholly. If therefore He be loved wholly by another, this one will love Him as much as God loves Himself.
But this is unreasonable. Therefore God cannot be wholly loved by a creature.
On the contrary, It is written (Deuteronomy 6:5): "Thou shalt love the Lord thy God with thy whole heart."
I answer that, Since love may be considered as something between lover and beloved, when we ask whether God can be wholly loved, the question may be understood in three ways, first so that the qualification "wholly" be referred to the thing loved, and thus God is to be loved wholly, since man should love all that pertains to God.
Secondly, it may be understood as though "wholly" qualified the lover: and thus again God ought to be loved wholly, since man ought to love God with all his might, and to refer all he has to the love of God, according to Deuteronomy 6:5: "Thou shalt love the Lord thy God with thy whole heart."
Thirdly, it may be understood by way of comparison of the lover to the thing loved, so that the mode of the lover equal the mode of the thing loved. This is impossible: for, since a thing is lovable in proportion to its goodness, God is infinitely lovable, since His goodness is infinite. Now no creature can love God infinitely, because all power of creatures, whether it be natural or infused, is finite.
This suffices for the Replies to the Objections, because the first three objections consider the question in this third sense, while the last takes it in the second sense.
Source: Summa Theologica (New Advent)