Mdd, the Chazon Ish’s point is gevaldig. Even if one was brought up frum why does that necessarily obligate him to keep mitzvos, if he later had some bad experience, or had other problems with his faith? How is he any different than a public school child, just because somebody told him he should keep Shabbos, or just because he kept it for a few years when he was younger? How does he really know that the mesorah is true?
The Chazon Ish is stating that until we get to see Nevuah with our own eyes, we can’t really be considered to fully know the emes, whether we were brought up frum or not. Once one has actually seen Nevuah, then one cannot deny that the Torah is emes, unless he is a real rasha.