Form of cleansing

[Quang Hai]

I am reading a story by William Trevor on the conflict between Catholic and Protestant in Northern Ireland (Lost ground). In a paragraph where author is writting on an old Protestant Preacher - Uncle Willie. He is great - uncle of a boy name Milton. Whole paragraph as below:

'Men still preached in the towns, at street corners or anywhere that might attract a crowd, but the preachers were fewer than they had been in the heyday of Milton's Great Uncle Willie because the popularity of television kept people in at nights, and because people were in more of a hurry. But during the days that followed the July celebration Milton remembered his great-uncle’s eloquence. He remembered the words he had used and the way he could bring in a quotation, and the way he was so certain. Often he had laid down that a form of cleansing was called for, that vileness could be exorcized by withering it out of existence.'

I could not understand the last sentence at all.
'Often he had laid down that a form of cleansing was called for, that vileness could be exorcized by withering it out of existence.'
1. What is form of cleansing? Uncle Willie laid it down for what?
2. I do not either understand meaning of 'that vileness could be exorcized by withering it out of existence.'. What is 'withering it out of existence'

please explain. thanks!

 

[Tdol]

Something like:

He had often emphasised that some sort of cleansing was necessary, that badness could be purged by reducing it to nothing.