If you were to travel back in time and ask a medieval peasant, ‘Why do you belong to the Church?’, he might look at you strangely. For the medievals, the Church was simply the air they breathed. Everyone they knew was Catholic. One might as well ask him why he was a Briton or why he was a man. He would be aware that non-Catholics existed in the form of heretical sects or pagan religions, but the thought of becoming one of them would never occur to him.
Contrast this with today: every Catholic in the West would understand the question, even if they could not give a convincing answer. We are all aware that being Catholic is a positive choice we make, and that there are many other options. One could become Lutheran or Muslim, or nothing at all.
