It's pretty straight forward. Decode the message, each letter is consistantly-(always) replaced-(maybe) by a different letter in the alphabet.
And don't forget the spoilers!
-But wait, there's a twist. The letter 'O' has been completely removed from the quote below. It is up to you to figure out the words and where the O's are missing. Good Luck!
P MLOS YSLECSD TPYSCHS QEN JMS JLYILJPOS, JYSELJPC QEN JMS PCJYSELCJ, LCD IPCDCSTT QEN JMS VCIPCD; BSJ, TJELCRS, P LN VCRELJSQVY J JMTS JSLHMSET. ~ILMYPY RPAELC