What does 2 Corinthians 6:1 mean?
The previous chapter ended with Paul begging his readers, on behalf of Christ, to be reconciled to God. The final verse made it abundantly clear that this does not happen through human effort. It is a gift of God's grace. God made sinless Jesus to be sin for the sake of humanity. God fully punished that sin through Jesus' death on the cross. Those who trust in Christ are given credit by God for Jesus' righteousness. They literally take their place "in Christ" in God's eyes.Paul now appeals to the Corinthians to not receive God's grace in vain. First, though, he describes himself and his co-workers as co-workers with God. Some hear this as a bold statement. How can anyone be God's co-worker? It can only happen by God's appointment. Paul has described himself and his team as "ambassadors for Christ" with God making his appeal to humanity through them (2 Corinthians 5:20). In this way, they work together with God as His representatives.
This raises the question of how anyone could receive God's grace "in vain." "In vain" means without purpose or without result. In this context, Paul is referring to the way the Corinthians respond to this amazing message. It's likely that some in Corinth believe themselves to be receiving God's grace when they are not. That could only be because they are not putting their faith in Christ in order to be forgiven of sin and be declared righteous by God. A person is not truly "in Christ" if they are not believing the true gospel.
False teachers had moved in among the Corinthians. It's possible these are Judaizers: those teaching that Christians must also follow the law in order to be truly saved. A person who trusts his own works to save him is not trusting Christ. That would be a false gospel (2 Corinthians 11:4).
It's also possible that Paul has in mind some among the Corinthians who continued to worship idols while participating in the Christian church. Again, such people would not be trusting in Christ if they believed they needed idols to provide for them, as well. They would be attempting to receive God's grace, but doing so in vain.