To be united to God through Christ takes place when all past sins are removed: Sin keeps man separated man from God. Holiness of heart unites man with God and begins at the new birth.
This operation is Biblically defined as reconciliation. All sins of the past are taken away. God no longer holds this man accountable for past deeds and conduct.
Salvation takes place. Justification takes place. Regeneration takes place.
A clean heart is given. A new life of holiness begins and is maintained by faith.
2 Cor. 5:18-19...And all things are of God, who hath reconciled us to himself by Jesus Christ, and hath given to us the ministry of reconciliation; To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them;
Reconciliation deals with the past and present life only. Man's future life is only determined by staying united with God in Christ, keeping the heart clean by the saving Grace of God, and remaining faithful to Christ...every minute, every hour, every day until you leave this world.