Why is Deuteronomy 29:29 important for understanding Paul's use of the word mystery?

Les teaches that Paul uses the word mystery as something intrinsic to his writings, and that this word comes from the Greek word mysterion, which simply means secret. The words are used synonymously or interchangeably throughout Paul's letters, so when Paul says "Behold, I show you a mystery," he is saying, in effect, "Behold, I show you a secret." Les likes to go back to Deuteronomy 29:29 especially whenever there is a reference to Paul's use of the word mystery, because God in His sovereignty has every right and every reason to keep things secret as long as He wants to keep them secret. But once He reveals it, He expects mankind to believe it and take it by faith. Les says this verse is so loaded when it comes to understanding what Paul says when he declares, "I'm revealing a secret."