In what sense did Paul know Christ after the flesh without having met him?
Paul knew Christ after the flesh as historical awareness, not personal acquaintance. Though Saul and Jesus never crossed paths, Saul, a contemporary and rising figure in Judaism, knew who Jesus was, what He was doing, where He came from, and all about Him. That earthly knowledge contrasts with knowing Christ now through His death and resurrection.
Les explains that Paul could speak of having known Christ after the flesh because Paul, as Saul of Tarsus, was about the same age as Christ. When Jesus began His earthly ministry at age 30, Saul was about the same age and was a young up-and-comer in Judaism. Even though Saul and Jesus never crossed paths, Saul knew who Jesus was, what He was doing, where He came from, and all about Him. Yet he did not truly know Him. So Paul says, yes, we knew Christ in the flesh, but henceforth we know Him no more. Les stresses that preaching only Jesus' earthly ministry is not enough, because there is no salvation in merely understanding it. One must go to the cross and the resurrection, and beyond, knowing Christ after the resurrection as the new creation.