What does Paul mean in 2Cor 6:1? Some things I think we can conclude: One, he is talking to the church, albeit the Corinthian church. He is talking to professing Christians so it must be that people who profess to be Christians can be in danger of receiving the grace of God in vain. Secondly, receiving the grace of God IN VAIN actually negates the grace of God. In other words, if someone receives God's grace in vain that means they did so in an empty, meaningless, powerless way so really that person did not truly receive the grace of God. That's why I say Paul was addressing PROFESSING Christians. He certainly was addressing the entire Corinthian church (2Cor 1:1) so, the third thing to note, is that it must be true a church can have a mixture of people who have truly received the grace of God (in power) and those who have "received" it in vain. Fourthly, one evidence of having received the grace of God in vain is that you don't separate yourself from the world (2Cor 6:14-7:1). That was how some of the Corinthians were demonstrating it so Paul warned them and pleaded with them in 6:1 to not be ones who receives the grace of God in vain. Some were apparently claiming the name of Christian but also maintaining their relationships, ties, activities and behaviors they had as non-Christians. They looked and acted just like pagans/unbelievers.
What a challenge to all who name the name of Christ! "Come out from among them" the Lord says. Paul challenged the Corinthians to examine themselves to see if they be in the faith (2Cor 13:5). We must do the same. If we wish to follow Christ and fellowship with Him we cannot follow and fellowship with "the world" or with sin. Oh Lord, change my heart and keep it tender and repentant so that I will not be one who received Your grace in vain!