27. This preparation will not lose sight of the importance of helping young people acquire a critical ability with regard to their surroundings, and the Christian courage of those who know how to be in the world without belonging to it. This is what we read in the Letter to Diognetus, a venerable and certainly authentic document from the early Christian era: "Christians are not distinguished from the rest of mankind by either country, speech, or customs...the whole tenor of their way of living stamps it as worthy of admiration and admittedly extraordinary... They marry like all others and beget children; but they do not expose their offspring. Their table they spread for all, but not their bed. They find themselves in the flesh, but do not live according to the flesh" (V, 1, 4, 6, 7, 8). Formation should arrive at a mentality and personality capable of not being led astray by ideas contrary to the unity and stability of marriage, thus able to react against the structures of the so-called social sin that "With greater or lesser violence, with greater or lesser harm, every sin has repercussions on the entire ecclesial body and the whole human family" (Apostolic Exhortation Reconciliatio et Paenitentia, 16). In the face of these sinful influences and so many social pressures, a critical conscience must be instilled.