A man cannot content himself with the certain knowledge that the Catholic Church represents the fullness of Christianity (supposing that he has that certainty unimpaired); he needs to know what 'the others' represent in relation to that Church and the salvation of which she is as it were the sacrament. In many places, every Catholic knows people who are Protestant, perhaps a communist or two, maybe a Jew, and certainly some who 'don't care', either because of indifference or because they are positively opposed to religion. These people are his fellows, sharing a common destiny with him, and he, or she, cannot but ask how they stand with regard to his religion, his faith, and that salvation in which he believes. The Communism pervading some countries, which has imposed some of its problems on us, is in its own way underlining the question: on the one hand by the atmosphere of human solidarity and ever-growing 'worldwideness' ; on the other, by consciousness of a tremendous historical continuity which forbids us to ignore the solidarity of generation with generation and century with century. It may be that the religion of the classical epoch was characterized by a certain individualism: Peter Nicole (d. 1695), for example, declared that 'A man is created to live alone with God for ever.' A possible comment on this nowadays is one that would have astonished and even scandalized Nicole: 'Save my own soul alone? No, it shall be all or none,' Whilst not going so far as that, this feeling for human solidarity certainly haunts many Christian consciences today.*
* This feeling could be particularly illustrated from the writings of Simone Weil, but there are much older expressions of it, though inspired by different considerations. There is Dostoevsky's theme that we are responsible for all people and everything, we have to beg forgiveness for all people and everything; and St Simeon the New Theologian (d. 1022) wrote; 'I knew a man who so longed for his brethren's salvation that, with excess of a zeal worthy of Moses, he implored God with scalding tears that either those brethren should be saved with him or he be damned with them. For he was bound to them in the Holy Spirit by such a bond of love that he did not want even to enter the kingdom of Heaven if it meant having to be separated from them' (Discourse 22; P.G., 120, 424-5)
Yves Congar, The Wide World My Parish: Salvation and its Problems (1962) p. 6