You, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross. He disarmed the rulers and authorities and put them to open shame, by triumphing over them in him. (Colossians 2:13-15)
The reason that union with Christ makes a great difference for the believer is that Christ achieved a decisive triumph over the devil at Calvary. He did not remove Satan from the world, but he disarmed him to the extent that the weapon of damnation was stripped from his hand.
He cannot accuse believers of unforgiven sin. Which is the only accusation that can destroy us. And therefore, he cannot bring us to utter ruin. He can hurt us physically and emotionally ? even kill us. He can tempt us and incite others against us. But he cannot destroy us.
The decisive triumph of Colossians 2:13-15 is owing to the fact that “the record of debt that stood against us” was nailed to the cross. The devil made that record his chief accusation against us. Now he has no accusation that can hold in the court of heaven. He is helpless to do the one thing he wants to do most: damn us. He can’t. Christ bore our damnation. The devil is disarmed.
Another way to say it is in Hebrews 2:14-15: “[Christ became human] that through death he might destroy the one who has the power of death, that is, the devil, and deliver all those who through fear of death were subject to lifelong slavery.”
Death is still our enemy. But it is defanged. The viper’s poison has been drained away. The deadly sting is gone. The sting of death was sin. And the damning power of sin was in the demand of the law. But thanks be to Christ who satisfied the law’s demand. “O death, where is your victory? O death, where is your sting?” (1 Corinthians 15:55).