Read Romans 6:1-10
After an individual is justified by faith in Christ, he discovers that he still has a sin nature. This gives him trouble, and he finds himself committing sins that he does not wish to commit. Soon he may become a believer who is dominated by sin.
What does God do about this? The solution to this problem is our identification with Jesus Christ in His death and resurrection. In this identification God sanctifies, or makes holy, the justified individual (a sinner saved by grace).
Whereas justification deals with the guilt of sin, sanctification (identification) deals with the power of sin in the life of the believer.
How does God sanctify, or make holy, a believer in daily experience? Romans 6 gives the answer. The answer is our union, or identification, with Christ.
Notice again that everything we have is because of Christ. Being in union with, or identified with, Christ is what is meant throughout the New Testament by the expression "in Christ." Being in Christ simply means that the believer has become one with Christ, or identified with Him.
Christ is not just a partner walking alongside the believer; He actually indwells the believer. Therefore, the believer is identified with Christ because Christ's life is in the believer.
"I am crucified with Christ: nevertheless I live; yet not I, but Christ liveth in me: and the life which I now live in the flesh I live by the faith of the Son of God, who loved me, and gave himself for me" (Gal. 2:20).