Union with Christ means...

The exhortation to righteousness in Romans 6 is predicated on one crucial premise: the transforming power of the gospel and the new obedience created by union with Christ. Christ is the sphere of holiness, righteousness and redemption (cf. 1 Cor. 1.30) and believers are emancipated from the old age of sin and death and are uniquely empowered by baptism into Christ to live their lives in complete service to God. This thought is expressed most aptly as the indicative and imperative of Pauline ethics; because you have been united to Christ, you need not offer your body into the service of sin. —The Saving Righteousness of God, page 147

<idle musing>
Amen! Good preaching! If we are united with Christ—which we are—then we are free from the necessity of sin. Why don't we see more of it? Because a. we don't here it preached or taught and b. most people don't really believe it. After all, aren't we sinners? No! We are saints and the righteousness of Christ—at least that's what the scripture says. So, whom are you going to believe? Scripture or doctrine? Your call, but I'm going with scripture!
</idle musing>