'Rightly Dividing the Word'
2 Cor 5:16 Therefore, from now on, we regard no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer.
Paul (Saul of Tarsus) had never met Jesus in the flesh, and his knowledge of Jesus was from the religious leaders who presented Him as a rebellious false teacher and false Messiah.
As a devout Jew and an upright and zealous Pharisee, he went all out to destroy this new deviate sect of Jesus, thinking that he was doing God's work.
Act 8:3; 9:1 As for Saul, he made havoc of the church, entering every house, and dragging off men and women, committing them to prison... Then Saul, still breathing threats and murder against the disciples of the Lord, went to the high priest..
Jews' Understanding of Jesus
Isa 53:4 Surely He has borne our griefs and carried our sorrows; yet we esteemed Him stricken, smitten by God, and afflicted.
The religious leaders taught Jews that Jesus was a rebel and heretic who claimed Himself as the Messiah. God punished Him by having Him crucified.
So Paul like the rest of the Jews believed that Jesus (in the flesh) was a rebel, and His sufferings and death on the cross was the judgment and punishment of God for being a heretic.
Since Paul was blinded by the lies of the religious leaders, he could not see Jesus as the Messiah coming as the Lamb of God, who offered Himself to be chastised for their transgressions and iniquities so that they could be at peace with God.
Isa 53:5 But He was wounded for our transgressions, He was bruised for our iniquities; the chastisement for our peace was upon Him, and by His stripes we are healed.
Taking 2Cor 5:16 in Context
2 Cor 5:15-16 and He died for all, that those who live should live no longer for themselves, but for Him who died for them and rose again. Therefore, from now on, we regard no one according to the flesh. Even though we have known Christ according to the flesh, yet now we know Him thus no longer.
After Paul's conversion, he realised that he was wrong in believing Jesus was a rebel and heretic, and therefore confessed that he would no longer have this wrong perception of Jesus (now we know Him this no longer)
Not only that, Paul promised that he would no longer judge anyone based on their conducts before their conversion (old man), because he had earlier erred in judging our Lord Jesus.
We, too, must not judge a brethren by the persecutions, problems, sufferings, pains and even death, that he is enduring. Jesus out of His love for us went through all these for our sake, which Paul thought were God's judgement on Him.
Paul taught that as a Christian, he is no longer the worldly, sinful and rebellious old man, but a new creation with the Holy Spirit in him. He should no longer live for himself but for our Lord Jesus, who died and redeemed him from the fire of hell, by keeping His words and commandments.
2 Cor 5:17 Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
Note - Do not revile, persecute nor judge any Christian based on his old conduct as a non believer, but rather regard him as a new creation in Christ.