Objection 1. It would seem that the love of charity stops at God and does not extend to our neighbor. For as we owe God love, so do we owe Him fear, according Deuteronomy 10:12: "And now Israel, what doth the Lord thy God require of thee, but that thou fear... and love Him?" Now the fear with which we fear man, and which is called human fear, is distinct from the fear with which we fear God, and which is either servile or filial, as is evident from what has been stated above (Question 10, Article 2). Therefore also the love with which we love God, is distinct from the love with which we love our neighbor.
Objection 2. Further, the Philosopher says (Ethic. viii, 8) that "to be loved is to be honored." Now the honor due to God, which is known as "latria", is distinct from the honor due to a creature, and known as "dulia." Therefore again the love wherewith we love God, is distinct from that with which we love our neighbor.
Objection 3. Further, hope begets charity, as a gloss states on Matthew 1:2. Now hope is so due to God that it is reprehensible to hope in man, according to Jeremiah 17:5: "Cursed be the man that trusteth in man." Therefore charity is so due to God, as not to extend to our neighbor.
On the contrary, It is written (1 John 4:21): "This commandment we have from God, that he, who loveth God, love also his brother."
I answer that, As stated above (17, 6; 19, 3; I-II, 54, 3) habits are not differentiated except their acts be of different species. For every act of the one species belongs to the same habit. Now since the species of an act is derived from its object, considered under its formal aspect, it follows of necessity that it is specifically the same act that tends to an aspect of the object, and that tends to the object under that aspect: thus it is specifically the same visual act whereby we see the light, and whereby we see the color under the aspect of light.
Now the aspect under which our neighbor is to be loved, is God, since what we ought to love in our neighbor is that he may be in God. Hence it is clear that it is specifically the same act whereby we love God, and whereby we love our neighbor. Consequently the habit of charity extends not only to the love of God, but also to the love of our neighbor.
Reply to Objection 1. We may fear our neighbor, even as we may love him, in two ways: first, on account of something that is proper to him, as when a man fears a tyrant on account of his cruelty, or loves him by reason of his own desire to get something from him. Such like human fear is distinct from the fear of God, and the same applies to love. Secondly, we fear a man, or love him on account of what he has of God; as when we fear the secular power by reason of its exercising the ministry of God for the punishment of evildoers, and love it for its justice: such like fear of man is not distinct from fear of God, as neither is such like love.
Reply to Objection 2. Love regards good in general, whereas honor regards the honored person's own good, for it is given to a person in recognition of his own virtue. Hence love is not differentiated specifically on account of the various degrees of goodness in various persons, so long as it is referred to one good common to all, whereas honor is distinguished according to the good belonging to individuals. Consequently we love all our neighbors with the same love of charity, in so far as they are referred to one good common to them all, which is God; whereas we give various honors to various people, according to each one's own virtue, and likewise to God we give the singular honor of latria on account of His singular virtue.
Reply to Objection 3. It is wrong to hope in man as though he were the principal author of salvation, but not, to hope in man as helping us ministerially under God. On like manner it would be wrong if a man loved his neighbor as though he were his last end, but not, if he loved him for God's sake; and this is what charity does.
Source: Summa Theologica (New Advent)