If we have been united with him in a death like his, we shall certainly be united with him in a resurrection like his.
One of the most important realities in the Christian life is that we have a union with Christ. Note the words “we shall certainly be united with him in a resurrection like his.” There is a union between Christ and Christians so that what happened to Christ is counted by God as happening to us. His death is our death. God establishes this union, as 1 Corinthians 1:30 says, “Because of [God] you are in Christ Jesus.” God establishes a union between believers and Christ, in a way that makes it fitting for him to count Christ’s death to be our death.
This death is something that happened in history, once for all. It is applied to us now through our faith, but since Christ died in history only once, and Romans 6:5 says we were united to that, our death happened, in God's way of seeing things, on the day Christ died.
Here the apostle Paul teaches that in Christ — that is, in our union with Christ that God established — we are dead to sin, meaning this: In our truest position and our truest identity, we are completely and finally dead to sin — both its guilt and its power. This is decisive, unrepeatable, and unchangeable. This is the foundation for all our warfare against sin, and all our progress in holiness.
The Christian life is an already and a not-yet experience of this identity in union with Christ. What happened to Christ Jesus historically and finally and unchangeably — and to us in him — is applied to us not all at once in its fullness, but some now completely, and some now progressively, and all fully in the age to come.
For example, we are already fully forgiven and acquitted and declared righteous and justified in our union with Christ by faith alone. And we are already delivered from slavery to sin, that is, from the power of sin as the defining direction of our lives. And we are already able, by faith, to grow more and more triumphant over sin in our daily life.
But we are not-yet perfected in our daily, earthly experience. We must fight the fight of faith and become in experience, by faith, what we are perfectly in our union with Christ. So, you are not left to live in your own strength today, but you are free to live in the power of believing God’s promises for what he will do for you because of your union with Christ.