If we are really united to Christ, it should change the way we live our lives.
Think about sin and the pursuit of sanctification
If we are really united to Christ, our sin becomes Christ's sin. We can't leave him behind to go off into our sin. We can't turn our Christianity on and off.
When we cheat, Christ cheats with us.
When we lie, Christ lies with us.
When we step over or on the poor...Christ is right there with us.
Our union to Christ provides the foundation for our pursuit of a holy life. We shouldn't lead good lives because we owe God something. We shouldn't even lead good lives because we love Jesus. We strive to live good lives because we're united to Jesus.
Sin also impacts the community, under this framework. If I'm united to Christ and you're united to Christ, my sin impacts you, through Christ. Sin is no longer private. Sin is no longer just mine.
But this also means that my growth and selflessness impacts the community. A small change in my life toward the better, even if no one around me notices, matters because I'm connected to the community through Christ.
Think about our call to mission and service
Christ cares about the weak and the broken. He loves the defenseless and the downtrodden. He carries what the Catholics call a "preferential option" for the poor.
Our union to Christ means that we are dragged with him into his activity. If Christ is at work in the world today, then we must be also. We can't hide in cathedrals, scurry behind pews and throw money at the poor. Christ our Saviour is out among them. And we must do likewise.
Evangelism, justice work, service, missions, engaging with the world...all of these exterior things are non-optional. If God is truly at work in the world and we are truly united to him through Christ, then we must join him in his work.
Apart from the theology of union, the call to engage in mission and service feels like employment. We do it because we have to, to pay the bills, to put food on the table. Lots of Christians treat these activities this way. Burdensome. But with the theology of union, there's a different spin on it, a more natural adjustment, like the way you adjust your stride and rhythm in a three-legged race or when riding a tandem bike.
Perhaps God could have saved us without uniting us to Christ. But what kind of salvation would that be? We would be debtors still, worse, slaves.
No, union is necessary if we are to be loved. And we are, very much, loved.