This is pretty hard because the style is so old sounding. The passage seems to be talking about how parents often try to prevent their children from becoming painters, even though the life of a painter is just as virtuous (according to Christianity) as an accountant's.
X showed with what folly
is a shortened form of
X showed with what folly it was to [action taken by someone]
Which in modern English is
X showed how foolish it was to [action taken by someone]
So I think the whole phrase means:
The event showed how foolish it was for the parents to try to suppress their child's desire to become a painter
Implying that the event showed that it was very foolish to try to suppress a child's desire to become a painter.
If you want to know why that idiom works that way, I'll try to explain:
Whenever you have a question like "how tall is John?", you can push the verb forward to get the implicit answer to that question. For example, you can take the question "How tall is John?" and make the implicit answer "how tall John is", which means "John's height". The following two are equivalent
I know John's height
I know how tall John is
The idiom "X showed [implicit answer]" implies the answer to the corresponding question is significant/noteworthy. For example:
John's test scores showed how smart he is
John's test scores showed that he is very smart
So basically the passage is using the above construction, with the implicit answer "with what folly it was to [blah]", which corresponds to the question "With what folly was it to [blah]", which in modern English is "How foolish was it to [blah]".