I answer that, As stated above (I-II, Q. 25, AA. 1, 2, 3), when we were treating of the passions, joy and sorrow proceed from love, but in contrary ways. For joy is caused by love, either through the presence of the thing loved, or because the proper good of the thing loved exists and endures in it; and the latter is the case chiefly in the love of benevolence, whereby a man rejoices in the well-being of his friend, though he be absent. On the other hand sorrow arises from love, either through the absence of the thing loved, or because the loved object to which we wish well, is deprived of its good or afflicted with some evil. Now charity is love of God, Whose good is unchangeable, since He is His goodness, and from the very fact that He is loved, He is in those who love Him by His most excellent effect, according to 1 John 4:16: "He that abideth in charity, abideth in God, and God in him." Therefore spiritual joy, which is about God, is caused by charity.
Reply Obj. 1: So long as we are in the body, we are said to be "absent from the Lord," in comparison with that presence whereby He is present to some by the vision of "sight"; wherefore the Apostle goes on to say (2 Cor. 5:6): "For we walk by faith and not by sight." Nevertheless, even in this life, He is present to those who love Him, by the indwelling of His grace.
Reply Obj. 2: The mourning that merits happiness, is about those things that are contrary to happiness. Wherefore it amounts to the same that charity causes this mourning, and this spiritual joy about God, since to rejoice in a certain good amounts to the same as to grieve for things that are contrary to it.
Reply Obj. 3: There can be spiritual joy about God in two ways. First, when we rejoice in the Divine good considered in itself; secondly, when we rejoice in the Divine good as participated by us. The former joy is the better, and proceeds from charity chiefly: while the latter joy proceeds from hope also, whereby we look forward to enjoy the Divine good, although this enjoyment itself, whether perfect or imperfect, is obtained according to the measure of one's charity. _______________________
SECOND ARTICLE [II-II, Q. 28, Art. 2]
Whether the Spiritual Joy, Which Results from Charity, Is Compatible with an Admixture of Sorrow?
Objection 1: It would seem that the spiritual joy that results from charity is compatible with an admixture of sorrow. For it belongs to charity to rejoice in our neighbor's good, according to 1 Cor. 13:4, 6: "Charity . . . rejoiceth not in iniquity, but rejoiceth with the truth." But this joy is compatible with an admixture of sorrow, according to Rom. 12:15: "Rejoice with them that rejoice, weep with them that weep." Therefore the spiritual joy of charity is compatible with an admixture of sorrow.
Obj. 2: Further, according to Gregory (Hom. in Evang. xxxiv), "penance consists in deploring past sins, and in not committing again those we have deplored." But there is no true penance without charity. Therefore the joy of charity has an admixture of sorrow.
Obj. 3: Further, it is through charity that man desires to be with Christ according to Phil. 1:23: "Having a desire to be dissolved and to be with Christ." Now this desire gives rise, in man, to a certain sadness, according to Ps. 119:5: "Woe is me that my sojourning is prolonged!" Therefore the joy of charity admits of a seasoning of sorrow.
On the contrary, The joy of charity is joy about the Divine wisdom. Now such like joy has no admixture of sorrow, according to Wis. 8:16: "Her conversation hath no bitterness." Therefore the joy of charity is incompatible with an admixture of sorrow.