Objection 1. It would seem that the spiritual joy which proceeds from charity cannot be filled. For the more we rejoice in God, the more is our joy in Him filled. But we can never rejoice in Him as much as it is meet that we should rejoice in God, since His goodness which is infinite, surpasses the creature's joy which is finite. Therefore joy in God can never be filled.
Objection 2. Further, that which is filled cannot be increased. But the joy, even of the blessed, can be increased, since one's joy is greater than another's. Therefore joy in God cannot be filled in a creature.
Objection 3. Further, comprehension seems to be nothing else than the fulness of knowledge. Now, just as the cognitive power of a creature is finite, so is its appetitive power. Since therefore God cannot be comprehended by any creature, it seems that no creature's joy in God can be filled.
On the contrary, Our Lord said to His disciples (John 15:11): "That My joy may be in you, and your joy may be filled."
I answer that, Fulness of joy can be understood in two ways; first, on the part of the thing rejoiced in, so that one rejoice in it as much as it is meet that one should rejoice in it, and thus God's joy alone in Himself is filled, because it is infinite; and this is condignly due to the infinite goodness of God: but the joy of any creature must needs be finite. Secondly, fulness of joy may be understood on the part of the one who rejoices. Now joy is compared to desire, as rest to movement, as stated above (I-II, 25, 1,2), when we were treating of the passions: and rest is full when there is no more movement. Hence joy is full, when there remains nothing to be desired. But as long as we are in this world, the movement of desire does not cease in us, because it still remains possible for us to approach nearer to God by grace, as was shown above (24, 4,7). When once, however, perfect happiness has been attained, nothing will remain to be desired, because then there will be full enjoyment of God, wherein man will obtain whatever he had desired, even with regard to other goods, according to Psalm 102:5: "Who satisfieth thy desire with good things." Hence desire will be at rest, not only our desire for God, but all our desires: so that the joy of the blessed is full to perfection--indeed over-full, since they will obtain more than they were capable of desiring: for "neither hath it entered into the heart of man, what things God hath prepared for them that love Him" (1 Corinthians 2:9). This is what is meant by the words of Luke 6:38: "Good measure and pressed down, and shaken together, and running over shall they give into your bosom." Yet, since no creature is capable of the joy condignly due to God, it follows that this perfectly full joy is not taken into man, but, on the contrary, man enters into it, according to Matthew 25:21: "Enter into the joy of thy Lord."
Reply to Objection 1. This argument takes the fulness of joy in reference to the thing in which we rejoice.
Reply to Objection 2. When each one attains to happiness he will reach the term appointed to him by Divine predestination, and nothing further will remain to which he may tend, although by reaching that term, some will approach nearer to God than others. Hence each one's joy will be full with regard to himself, because his desire will be fully set at rest; yet one's joy will be greater than another's, on account of a fuller participation of the Divine happiness.
Reply to Objection 3. Comprehension denotes fulness of knowledge in respect of the thing known, so that it is known as much as it can be. There is however a fulness of knowledge in respect of the knower, just as we have said of joy. Wherefore the Apostle says (Colossians 1:9): "That you may be filled with the knowledge of His will, in all wisdom and spiritual understanding."
Source: Summa Theologica (New Advent)