When would Peter understand what Jesus was doing?
Peter would understand what Jesus was doing at a future day, when God would reveal it to him. Jesus told Peter plainly that although he did not know then what Jesus was doing, he would understand later, at the time God chose to reveal it.
Les explains that Peter was not yet in a position to understand all that Jesus was doing. Jesus plainly told him, 'You don't know what I'm doing, but you will at a future day when God would reveal it.' That future day would come when God revealed it, not immediately. Les connects this to the revelation of the mystery, the secret kept hidden, which Paul later writes about in Romans 16:25.
More Questions
What did Jesus tell Peter in John 13:7 about when he would understand his actions?
In John 13:7 Jesus tells Peter that he does not understand what Jesus is doing now, but he will understand at a later time.