When are we united with Christ as a child of God?
The moment a person becomes a child of God, they are literally united with Christ, just as a husband and wife are joined in marriage. This union is by the work of the cross, through which believers are made dead to the law and married to the risen Christ.
Les explains that the believer’s union with Christ occurs as soon as we become a child of God—at that moment we are literally unioned with Christ, just like a husband and wife in the marriage relationship. The context contrasts law and grace: by the work of the cross we are now under grace, not under the law, and we have become dead to the law by the body of Christ so that we should be married to another, to Him who is raised from the dead. Ephesians 5 likens the marriage relationship to our spiritual relationship with Christ. Christ is the head of the Church, and to become head of you and I as members of the Body, He had to die and suffer for us; the power of the resurrection overcame that death.
Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God.