Galatians 2:20, “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.”
Galatians 2:20 is a verse many believers know well. It’s one many of us have memorized. And for a good reason. It’s a significant verse with rich gospel implications. This verse reminds us that the gospel doesn’t merely change the course of our status in eternity; it transforms our lives in the here and now.
And it transforms us through our union with Christ.
United with Christ
When Paul talks about being crucified with Christ and describes Christ living in him, he’s referring to his spiritual union with Christ. The doctrine of union with Christ is a foundational truth regarding our salvation. It’s the source out of which we live the Christian life.
There are two aspects to this union. The first involves Christ’s representative work for us on our behalf. Christ united himself to us in our humanity by taking on human flesh. He then lived the life we could not live and died the death we deserved. Through faith in who Christ is and what he has done for us, we are united to him. All that he has done becomes ours. God looks at us and sees Christ’s righteousness. He sees Christ’s atoning death in our place and accepts it as though we paid it ourselves.
The second aspect to this union is the Spirit of Christ, who now resides within us. Upon salvation, we receive the gift of the Spirit, who takes up residence in our heart (2 Corinthians 1:22). He encourages, transforms, challenges, convicts, and trains us in righteousness.
This means that all we think of in terms of our salvation is rooted in our union with Christ. When Paul writes about this union, he often uses the preposition “in.” Our justification is in Christ; our sanctification is in Christ; our future glorification is in Christ. As Calvin wrote, “our whole salvation and all its parts are comprehended in Christ.” 
Spiritually speaking, we are in Christ, and he is in us. That’s why Paul could say that he was crucified with Christ. All that Christ did at the cross is credited to us. And because we are united to him by faith, our sins were crucified with him.
New Life in Christ
In Christ, our old selves have been put to death at the cross, and we have risen to new life in him. As Paul wrote elsewhere, “Therefore if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come” (2 Corinthians 5:17). Like those dry bones in Ezekiel, God has breathed new life into us. He gave us the gift of the Spirit, who lives within us. He has replaced our hearts of stone with a heart of flesh (Ezekiel 36). We can now love, obey, and worship God as we were created to.
We are not who we once were. We are now in Christ.
And so, we are to live in Christ. Just as our justification came to us by grace through faith, so too do we grow in faith through the grace of God. Just as we began, so do we continue. As Paul says, the Christian life he leads is “by faith in the Son of God.” We live out our Christian life through the power of the gospel and the Spirit at work in us. We don’t do it in our own strength or wisdom. We don’t do so in our own righteousness. We live out the Christian life in Christ.
Part of this new life means that we have a new identity: We are “in Christ.” All that Christ is and all that he has is ours. As Wilbourne Rankin wrote, “…being in Christ is to discover our true, God-given identity. You are alive in him, moving with him through this world, clothed in all his benefits and blessings. You are in Christ.”
This new identity transforms everything. When we are tempted to sin, we remember who we are because of what Christ has done. We remember that we are no longer slaves to sin; we’ve been set free to live for Christ. When we feel weak and insufficient, or lack the wisdom we need, we remember we have the Spirit of Christ living within us. He strengthens us by his grace and gives us his wisdom. When memories of past sin overshadow us, and we are filled with shame, we remember that because of Christ, God remembers our sins no more. He looks at us and sees the perfect life of Christ.
We are no longer who we once were. Our old selves have been crucified with Christ. We now live new lives, united to Christ through his work for us and by his Spirit within us. What a glorious truth! May we all memorize Paul’s words in Galatians 2:20 and rejoice in our union with Christ.
 Calvin’s Institutes, II.xvi.19.
 Wilbourne, Rankin Union With Christ p.48.