"Can you really, Jack, think of the Crusades or The Holy Office (the Spanish Inquisition which, notoriously, nobody expects!), and deny the proposition that "faith can be very dangerous" ['can be', note; NOT 'is']?"

Please do me the courtesy of addressing the entire quote.

"Faith can be very very dangerous, and deliberately to implant it into the vulnerable mind of an innocent child is a grievous wrong."

I think "can be" is not nearly enough to justify this. "deliberately to implant it into the vulnerable mind of an innocent child is a grievous wrong."

Is he saying that Joe should not have sent his kids to Catholic school because they might have grown up to burn people as witches?

You think that maybe that your argument is a few hundred years too late?