Under Covenant, the twelve apostles were SORRY about the death of Christ and called it a murder.
Matthew 17:22-23 And while they abode in Galilee, Jesus said unto them, The Son of man shall be betrayed into the hands of men: And they shall kill him, and the third day he shall be raised again. And they were exceeding sorry.
Acts 3:14-15 But ye denied the Holy One and the Just, and desired a murderer to be granted unto you; And killed the Prince of life, whom God hath raised from the dead; whereof we are witnesses.
Under Grace, not only was Paul NOT sorry, he GLORIED in the death of the savior.
Galatians 6:14 But God forbid that I should glory, save in the cross of our Lord Jesus Christ, by whom the world is crucified unto me, and I unto the world.
Paul, like the twelve apostles, would have also been sorry if he did not understand the fullness of what Christ accomplished on the cross. The information Christ, gave to Paul on this side of Calvary was "unsearchable" (Ephesians 3:8) before being given to him. An important piece of unsearchable data not made known to the twelve apostles is found in 1 Corinthians 2:8.
“Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.”
The “world” including Satan, had no clue of what Christ was going to accomplish at the cross, and if Satan WOULD have known, he “would not have crucified the Lord of glory.”
Hence the reason for a new-apostle with new-information after the cross. One who was selected to write down and reveal “the mystery, which from the beginning of the world hath been hid in God,” (Ephesians 3:9).
It was Paul that Christ chose to reveal "the mystery" of the cross to, and it's Paul's gospel that has the power to stablish us.
"Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began," (Romans 16:25).
"A half truth is a whole lie."