Paul, the apostle certainly knew that Jesus Christ was crucified FOR him. The fact that Jesus died FOR us – tasted death for every person – is an absolute fact and the foundation of the redemption. But it is precisely because Christ died FOR us that Paul was able to make this statement:
I am crucified with Christ. (Gal. 2:20)
If Jesus merely died FOR us, then we could be legally forgiven. But our old man in Adam would not be crucified in Him. The body of sin would not be destroyed. (Rom. 6:6) That means we would remain in bondage to the old. And there would be no new life possible.
Only if I am crucified WITH Christ does a death take place in me regarding my old man in Adam. Otherwise, His death has no impact upon me other than a legal means of forgiveness.
Because Jesus died for us, we can DIE IN HIM. And only if we die in Christ can we be raised in Him. This is the basis for all freedom from sin, and victory over Satan. True faith will therefore embrace this Truth and realize that, yes, Christ died FOR us, but we are also, “crucified WITH Christ.” It is finished.