24 Let us ask once again: what may we hope? And what may we not hope? First of all, we must acknowledge that incremental progress is possible only in the material sphere. Here, amid our growing knowledge of the structure of matter and in the light of ever more advanced inventions, we clearly see continuous progress towards an ever greater mastery of nature. Yet in the field of ethical awareness and moral decision-making, there is no similar possibility of accumulation for the simple reason that man's freedom is always new and he must always make his decisions anew. These decisions can never simply be made for us in advance by others—if that were the case, we would no longer be free. Freedom presupposes that in fundamental decisions, every person and every generation is a new beginning. Naturally, new generations can build on the knowledge and experience of those who went before, and they can draw upon the moral treasury of the whole of humanity. But they can also reject it, because it can never be self-evident in the same way as material inventions. The moral treasury of humanity is not readily at hand like tools that we use; it is present as an appeal to freedom and a possibility for it. This, however, means that: a ) The right state of human affairs, the moral well-being of the world can never be guaranteed simply through structures alone, however good they are. Such structures are not only important, but necessary; yet they cannot and must not marginalize human freedom. Even the best structures function only when the community is animated by convictions capable of motivating people to assent freely to the social order. Freedom requires conviction; conviction does not exist on its own, but must always be gained anew by the community. b ) Since man always remains free and since his freedom is always fragile, the kingdom of good will never be definitively established in this world. Anyone who promises the better world that is guaranteed to last for ever is making a false promise; he is overlooking human freedom. Freedom must constantly be won over for the cause of good. Free assent to the good never exists simply by itself. If there were structures which could irrevocably guarantee a determined—good—state of the world, man's freedom would be denied, and hence they would not be good structures at all.
Source: Spe Salvi (Vatican.va)