Sorry, but this is a fake. I’m a big C.S. Lewis fan. Lewis never wrote a book entitled “The Devil’s Letter to His Nephew.” He did write “The Screwtape Letters,” which is about the devil writing to his nephew, but the quote that is cited in the Facebook page does not appear in that book, or anything close to the quote in any of his writings. A quick google search will verify this, such as:
http://www.essentialcslewis.com/2020/06/20/ccslq-55-another-fake-screwtape-quote-souls-to-hell/
Sorry, it seemed a little too “prophetic.” I guess it shows how easily one can be duped on the internet.