The World's Fulfillment
"Thou hast made us for thyself, and our heart is restless until it rests in thee."
Augustine of Hippo, Confessions, Book 1 (397 CE). Augustine was thinking of the morbid restlessness of alienation from God, but his words also apply to the healthy restlessness of striving toward God.
"[God] has made known to us ... his purpose ... to unite all things."
Paul's Epistle to the Ephesians, 1:9.
We are limited but our aspiration is boundless. Thus we live in tension. Some see this as a problem but we see it as a pointer to our capacity for union with God and our destiny in that union. He, our source, is also our proper final goal. Only he can satisfy the minds he has created. Every mind has "a God-shaped gap", and if that is what we need it would be absurd to doubt that God will in due course supply it.
Whoever you are, there are things you look to to make your life worth living. These are your gods, declared by your actions, whatever your words and feelings may say. What are your gods? Family? Sport? Art? Politics? Power? Peak experiences induced by drugs, danger, venery or meditation? The list could be extended. Some of these are valuable, but if you examine your own mind carefully you will find that none of them is satisfactory. We need to look beyond all such things, to something shared by all, to God.
But we are not separate from our world. Our souls are not detachable from our bodies. We are not visitors here. This is our world and we're staying right here, in life, death and resurrection. We have no fulfillment apart from our world. The fulfillment of Humanity is the fulfillment of Humanity's world.
Objections
"L'Homme est une passion inutile." ("Man is a futile self-sacrifice.")
Jean-Paul Sartre, Being and Nothingness (1943), at the end of 4.2.3.
Sartre's (French) passion is sometimes rendered as English "passion", but this is misleading. In French as in older English, it means "suffering" not "enthusiasm", and Sartre was referring specifically to the kind of voluntary suffering epitomised by Christ's self-sacrifice. Sarte compares man's perennial quest for transcendence with Christ's voluntary suffering. He claims that this quest must be regarded as futile because we must reject the idea of God. He is right that without God man is futile. Happily, we are not without God!