Why did Jesus tell Peter he would understand later?
Peter was not yet in a position to understand what Jesus was doing. Jesus explained that Peter did not know then, but would understand at a future day when God revealed it. The timing depended on divine revelation, not Peter’s present capacity.
Les explains that Peter was not in a position to understand all this as yet. So Jesus plainly told him that he did not know what Jesus was doing, but he would at a future day when God would reveal it. The point is that understanding would come by divine revelation at a future day, not by Peter’s own insight in the moment. The surrounding context later points to Romans 16:25, where Paul speaks of the preaching of Jesus Christ according to the revelation of the mystery, or secret—Greek mysterion, translated secret—which had been kept secret.