Why did Paul stress 'we know Christ no more' in 2 Corinthians 5:16?

Les explains that it is good to know the miracles Christ did among the Jews, what led to His crucifixion, and how people responded to the resurrection in John 20 and 21, but that is not the Gospel. The Gospel comes out where Paul comes on the scene: Christ commended His love toward us, in that while we were yet sinners, Christ died for us, and He gave Himself for us. That is all the man knows—the preaching of the cross. So Paul stresses in verse 16, “though we have known Christ after the flesh, yet now henceforth we know Him no more.” Les suggests Paul and Jesus were almost identically the same age; Jesus was 30 when He began His earthly ministry and 33 when crucified, while Saul of Tarsus was a key man of the Sanhedrin, no doubt in his 30s. As Saul heard about Jesus’ miracles and Jews following the Messiah from Nazareth, it infuriated him as he defended his religion, and later his apostleship.