48 The Church well knows that no temporal achievement is to be identified with the Kingdom of God, but that all such achievements simply reflect and in a sense anticipate the glory of the Kingdom, the Kingdom which we await at the end of history, when the Lord will come again. But that expectation can never be an excuse for lack of concern for people in their concrete personal situations and in their social, national and international life, since the former is conditioned by the latter, especially today. However imperfect and temporary are all the things that can and ought to be done through the combined efforts of everyone and through divine grace, at a given moment of history, in order to make people's lives "more human," nothing will be lost or will have been in vain. This is the teaching of the Second Vatican Council, in an enlightening passage of the Pastoral Constitution Gaudium et Spes: "When we have spread on earth the fruits of our nature and our enterprise - human dignity, fraternal communion, and freedom - according to the command of the Lord and in his Spirit, we will find them once again, cleansed this time from the stain of sin, illumined and transfigured, when Christ presents to his Father an eternal and universal kingdom...here on earth that kingdom is already present in mystery." The Kingdom of God becomes present above all in the celebration of the sacrament of the Eucharist, which is the Lord's Sacrifice. In that celebration the fruits of the earth and the work of human hands - the bread and wine - are transformed mysteriously, but really and substantially, through the power of the Holy Spirit and the words of the minister, into the Body and Blood of the Lord Jesus Christ, the Son of God and Son of Mary, through whom the Kingdom of the Father has been made present in our midst. The goods of this world and the work of our hands-the bread and wine-serve for the coming of the definitive Kingdom, since the Lord, through his Spirit, takes them up into himself in order to offer himself to the Father and to offer us with himself in the renewal of his one Sacrifice, which anticipates God's Kingdom and proclaims its final coming. Thus the Lord unites us with himself through the Eucharist- Sacrament and Sacrifice-and he unites us with himself and with one another by a bond stronger than any natural union; and thus united, he sends us into the whole world to bear witness, through faith and works, to God's love, preparing the coming of his Kingdom and anticipating it, though in the obscurity of the present time. All of us who take part in the Eucharist are called to discover, through this sacrament, the profound meaning of our actions in the world in favor of development and peace; and to receive from it the strength to commit ourselves ever more generously, following the example of Christ, who in this sacrament lays down his life for his friends (cf. Jn 15:13). Our personal commitment, like Christ's and in union with his, will-not be in vain but certainly fruitful.
Source: Sollicitudo Rei Socialis (Vatican.va)