More than 20 years later, I recognize the truth of this rule. To grow in faith it is necessary to remember how faith came to me. If Christ makes himself present through the Church, through the baptized, then an encounter with them is in some way an encounter with Him, no matter if they are aware of it, no matter if their theology is defective. Every detail of the encounter is important: what time (4 in the afternoon?), what were the circumstances, who was there, what was said. It is possible to remember the encounter because the encounter is never merely interior, any more than anything in human life is purely interior. It's possible to remember the encounter because it happens again and again. It repeats, reverberates, resounds. It deepens, brings to life, and grows like yeast in bread – or like the tiniest seed. Heaven and earth will pass away but my word will not pass away.
An interview with Msgr. Giussani is newly available online. Here's one question and the culmination of an answer:
Q. We spoke about Protestant culture and Orthodox as well. Since you have such a lively sympathy for these religious traditions, why are you Catholic?
Giussani: [....] "It is precisely the respectful and admiring encounter I had with the spirit of Protestantism and the genius of Orthodoxy that allowed me to better understand how the Catholic Church is the only place where the Orthodox sense of communion and the Protestant zest for the concrete and for the individual can be harmoniously reconciled in a complete synthesis."