How do you explain the enormous success of Opus Dei, and by what criteria do you measure this success?

When an undertaking is supernatural, its 'success' or 'failure' in the ordinary sense of the word is relatively unimportant. As Saint Paul said to the Christians at Corinth, what matters in the spiritual life is not what others think of us, or even our own opinion of ourselves, but God's opinion.

Undoubtedly the Work has spread all over the world. Men and women of close to seventy nationalities now belong to it. To tell the truth, it is something that surprises me. I cannot provide any explanation for it. The only explanation is the will of God, for 'the Spirit breathes where He will' and He makes use of whomever He sees fit to sanctify men. For me it is an occasion for thanksgiving, for humility and for asking God for the grace to serve Him always.

You also asked by what criteria I measure and judge. The answer is very simple: sanctity, fruits of sanctity.

Opus Dei's most important apostolate is the testimony of the life and conversation of each individual member in his daily contacts with his friends and fellow workers. Who can measure the supernatural effectiveness of this quiet and humble apostolate? It is impossible to evaluate the help we receive from a loyal and sincere friend or the influence of a good mother over her family.

But perhaps your question refers to the corporate apostolates carried out by Opus Dei, supposing that their results can be measured from a human or technical viewpoint: whether a technical training centre for workers contributes to the social advancement of its pupils, whether a university offers its students an adequate cultural and professional formation. If that was your intention, I would say that their results can be explained in part by the fact that they are undertakings carried out by carefully trained professionals who are practising their own profession. This implies, among other things, that these activities are planned in every case in the light of the particular necessities of the society in which they are to be carried out, and adapted to real needs, not according to preconceived schemes.

But let me repeat that Opus Dei is not primarily interested in human effectiveness. The real success or failure of our activities depends on whether, in addition to being humanly well-run, they help those who carry them out and those who make use of their services to love God, to feel their brotherhood with their fellow men, and to manifest these sentiments in a disinterested service of humanity.