1. Vatican I definitively teaches that the world is created for the glory of God (see DS 3025/1805).2 The glory of God primarily is his very divinity and perfection. But his intrinsic glory as supreme being is manifested outwardly, like the light that streams from the sun (see Is 60.1–3; Rv 21.23). It is realized in the minds of intelligent creatures, who recognize what God is and does, and appreciate and praise him (see Eph 1.11–14).3
2. But God does not create to acquire praise and honor for himself; he does not use us for his fulfillment. While Vatican I is anxious to exclude the idea that human happiness is the be-all and end-all of creation—which would make God a mere servant of the fulfillment of creatures—it also holds that God creates “to manifest his perfection through the goods which he makes creatures share in, not to increase his happiness nor to acquire anything” (DS 3002/1783; translation supplied). To suppose God gains anything at all by creating us would be to suppose God needs us, in which case he would not be God.
3. God depends on nothing else, and his actions must be understood as motivated ultimately by his love of his own, fully actual goodness. However, God also knows that his goodness can be manifested, expressed, communicated, and shared with creatures. His free choice to create the universe, including ourselves, is thus an act of pure generosity.
4. Plainly, the whole universe is the greatest created good, because it is the fullest created expression of God’s goodness. Human fulfillment is only a part of this whole and, as such, not ultimate. But it does not follow that God uses us for an ulterior purpose; rather, we and our fulfillment are important parts of the self-expression God intends in creating (see S.t., 1, q. 44, a. 4; q. 47, a. 1; q. 65, a. 2; S.c.g., 3, 20–22).
5. In sum, the purpose of the whole of creation is divine goodness, considered insofar as it can be expressed in creation. Our fulfillment is to be like God, to manifest his goodness in our being and actions. We are called to live for God’s glory, not merely our own happiness. This is not because God is using us, but because our happiness is only part of that larger expression of God’s goodness which is the whole of creation.
6. Therefore, rather than being alternative purposes, God’s glory and human fulfillment are inseparably joined. Whatever takes away from human dignity takes away from God’s glory, because it takes away from the expression of his goodness (see S.c.g., 3, 69).
God, perfect in himself, creates all things not to acquire anything, but solely to express his goodness (see S.t., 1, q. 19, aa. 2–3; q. 20, aa. 1–2; q. 44, a. 4; S.c.g., 1, 74, 83). God creates by a completely free decision, cares for and blesses all things he makes, preserves them in being, and orders them with gentleness (see DS 3001–3/1782–84). Reflecting upon God’s power, the book of Wisdom finds in it the ground of his mercy and willingness to forgive. God’s mercy is absolutely universal:
For you love all things that exist,
The wonder of creation—the coming forth of things from God by his simple word—leads us to marvel at God’s power: We believe in God, the Father almighty, creator of heaven and earth. But the total gratuity of the creative act, God’s utter freedom in choosing to create, his generosity in blessing his creatures, his constant care for all things—all these make it clear that creation is the work and expression of God’s love.
and have loathing for none of the things which you have made,
for you would not have made
anything if you had hated it.
How would anything have endured
if you had not willed it?
Or how would anything not called forth
by you have been preserved? (Wis 11.24–25)
This is an important matter, which deeply affects our attitude toward God and shapes the spirit in which we live our lives for his glory. If we have any sense of being used, we will be resentful. Only if we correctly understand what it means to say the world is made for God’s glory will we realize that in acting for the glory of God we are freely cooperating, by works which are themselves his gifts, in receiving from God a share in his own perfection. And only if we realize this truth will we feel toward God the wonder and gratitude we ought to feel, and so be able to live with the dedication and joy of true followers of Jesus.
2. Two important articles by one author clarify many of the points considered in this question: Philip J. Donnelly, S.J., “St. Thomas and the Ultimate Purpose of Creation,” Theological Studies, 2 (1941), 53–83; “The Doctrine of the Vatican Council on the End of Creation,” Theological Studies, 4 (1943), 3–33.
3. On the concept of glory in Scripture and its place in the divine plan for creation, see S. Aalen, “doxa,” New International Dictionary of New Testament Theology, 2:44–48.