The heart of Paul’s religion is union with Christ. This, more than any other conception — more than justification, more than sanctification, more even than reconciliation — is the key which unlocks the secrets of his soul.
Within the Holy of Holies which stood revealed when the veil was rent in twain from the top to the bottom on the day of Damascus, Paul beheld Christ summoning and welcoming him in infinite love into vital unity with Himself.
If one seeks for the most characteristic sentences the apostle ever wrote, they will be found, not where he is refuting the legalists, or vindicating his apostleship, or meditating on eschatological hopes, or giving practical ethical guidance to the Church, but where his intense intimacy with Christ comes to expression. Everything that religion meant for Paul is focused for us in such great words as these : “I live, yet not I, but Christ liveth in me” (Galatians 2:20), “There is, therefore, now no condemnation to them which are in Christ Jesus.” (Romans 8:1), “He that is joined unto the Lord is one spirit.” (I Corinthians 6:17).
From “A Man in Christ” by James Stewart