Objection 1. It would seem that charity is not one virtue. For habits are distinct according to their objects. Now there are two objects of charity--God and our neighbor--which are infinitely distant from one another. Therefore charity is not one virtue.
Objection 2. Further, different aspects of the object diversify a habit, even though that object be one in reality, as shown above (17, 6; I-II, 54, 2, ad 1). Now there are many aspects under which God is an object of love, because we are debtors to His love by reason of each one of His favors. Therefore charity is not one virtue.
Objection 3. Further, charity comprises friendship for our neighbor. But the Philosopher reckons several species of friendship (Ethic. viii, 3,11,12). Therefore charity is not one virtue, but is divided into a number of various species.
On the contrary, Just as God is the object of faith, so is He the object of charity. Now faith is one virtue by reason of the unity of the Divine truth, according to Ephesians 4:5: "One faith." Therefore charity also is one virtue by reason of the unity of the Divine goodness.
I answer that, Charity, as stated above (Article 1) is a kind of friendship of man for God. Now the different species of friendship are differentiated, first of all, in respect of a diversity of end, and in this way there are three species of friendship, namely friendship for the useful, for the delightful, and for the virtuous; secondly, in respect of the different kinds of communion on which friendships are based; thus there is one species of friendship between kinsmen, and another between fellow citizens or fellow travellers, the former being based on natural communion, the latter on civil communion or on the comradeship of the road, as the Philosopher explains (Ethic. viii, 12).
Now charity cannot be differentiated in either of these ways: for its end is one, namely, the goodness of God; and the fellowship of everlasting happiness, on which this friendship is based, is also one. Hence it follows that charity is simply one virtue, and not divided into several species.
Reply to Objection 1. This argument would hold, if God and our neighbor were equally objects of charity. But this is not true: for God is the principal object of charity, while our neighbor is loved out of charity for God's sake.
Reply to Objection 2. God is loved by charity for His own sake: wherefore charity regards principally but one aspect of lovableness, namely God's goodness, which is His substance, according to Psalm 105:1: "Give glory to the Lord for He is good." Other reasons that inspire us with love for Him, or which make it our duty to love Him, are secondary and result from the first.
Reply to Objection 3. Human friendship of which the Philosopher treats has various ends and various forms of fellowship. This does not apply to charity, as stated above: wherefore the comparison fails.
Source: Summa Theologica (New Advent)