The Corinthian Christians had received God's grace. By which Paul means:

That they had been reconciled to God through receiving the death and resurretion of Jesus on their behalf. They have become new creations (5:17)
That God had made Jesus sin for them, and them the righteousness of God in him (5:21)

But now Paul is saying that they may have received all of this in vain. How is it possible to receive grace in vain? The clue is in the connection between the first two verses of chapter 6, where he says "don't receive grace in vain because the day of God's great, eternal, eschatalogical salvation has arrived. The time promised in the famous Isaiah 49:8 when the long-awaited Servant of the Lord will be - in and of himself - a new covenant for the people.

Paul has said a lot about the new covenant already. But here he homes in on a passage of the Old Testament that is overwhelmingly important for getting to grips with it. If you don't know it, you must get to know. It is seminally important. Briefly, what it prophecies is that God's anointed rescuer -who in the next chapter 53 will die an atoning, substitutionary death for sinners - himself becomes the new covenant. The old covenant was an agreement written on stone at Sinai. That's how God related to the people and them to him. The new covenant is the servant of the Lord. That is now how people relate to God and God to them - by Jesus Christ.

That is the day Paul says has dawned. That is what they are in danger of having received in vain. How is that possible? By finding ways to ignore it, so that it doesn't actually make any difference in your life, that's how. Paul says that the most startling, shocking, thrilling, amazing time has come, and it seems that they want to carry on as if it hasn't. We can see at least three ways or reasons advanced to simply carry on as before in the surrounding verses:

We don't think it is worth sacrificing ourselves and our comfort for (5:1-10, see previous post)
Let's disregard it because we think Paul is a bit rubbish (6:3-13)
If we are actually changed by this, and we really do have to live pure now, then it will disrupt our lives, reorient our friendships and change relationships with all our pals and colleagues who aren't believers (6:14-7:1). Much easier to carry on as before, just like all our idol-worshipping peers...

Here is the main challenge. If the day of salvation - the day of the Servant of the Lord, the time of God's favour - has really dawned, everything is different now. If you have received him you are different as light is from darkness (6:14). You have been transformed. It is impossible not to be different. If you aren't any different to before then you are showing that you haven't received it. If you don't want to be transformed, if you have no appreciable hunger for God, if you don't want to make him your delight then you received it grace in vain. It is useless to you.

People who identify as Christians but who have no wish to be changed by God, who have no desire to be any different to all the unbelievers around about them - who don't want to live their whole lives for Jesus, who simply don't care much about living in the light and spreading the light of the good news - probably aren't believers. They are maybe God-fearers or people who enjoy the environment and care created by Christians, or people who want a "spiritual bit" in their lives or some kind of divine insurance policy for when they die. But probably they are not believers. And there are loads of them in churches.

Minsters, pastors, those of us who lead and preach, we need to realise that there are people sitting in front of us every week, self-identifying as Christians who aren't. They don't need to be discipled, they need to be evangelised. And there are few tougher defences to break down tha those of people who think they are alright merely because they are faithful in attending a service every week. And for whom there is no other discernible new covenant difference. Ask people "what does Jesus mean to you?" or "what is God doing in your life at the moment?" or "how are you enjoying God and being his child?" and you will soon discover which is which.

Some people in our congregations are receiving God's grace in vain.