2 Cor. 6  Verse 1 "And working together with Him, we also urge you not to receive the grace of God in vain -"  The Bible Knowledge Commentary indicates that the way this might happen - receiving God's grace in vain - is either to disbelieve the Gospel or to follow the teachings of false apostles, probably the Judaizers.  The message of the Gospel that Paul preached was salvation through faith in Jesus Christ alone; that is the gift ( grace)  of His righteousness exchanged for our sin.. through Jesus.

Paul wants the Corinthians to see the proof that his ministry is the truth; to not follow empty, useless, false teachings.  The proofs he gives are:
His endurance
His afflictions
His hardships
His distresses
His beatings, imprisonments, tumults, labors, sleeplessness, and hunger;
His purity, knowledge, patience, kindness,  - the fruits of the Spirit;
His genuine love;
The Word of truth, the power of God, and the weapons of righteousness -

All that they could readily see at work in Paul's life, the good and the bad, gave proof to Paul's genuine faith in Jesus Christ.  His life was God's message to them.  " .. by glory and dishonor, by evil report and good report;.. " Christ is seen in Paul's ministry.  He made every effort to live a life that gave glory to God.  They could see that for themselves if they would "open wide" their hearts to him, just as he had opened his heart to them.

Paul cautions the church about being "bound together with unbelievers".  Whether this refers to the false teachers or those who were still bound to idols - the presence of these people in the church would be allowing Satan himself ( Belial)  into God's temple.  They.. and we... are called to come out and be separate from those who are "unclean" - impure, morally corrupt, and even demonic.

"Therefore, having these promises beloved, ( the promises that God would walk among us and make us His people, that He would be a Father to us and we would be His sons and daughters. Verses 16-18).. let us cleanse ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of God." (7:1)

Father in heaven, we honor You and bless Your Name forever and ever.  There is no other god and no other name that we want to even speak.  We desire to live in holiness, clinging to You, our Father and our God.  It is through the gospel of Jesus Christ that we have been made Your children, exchanging our sins for His righteousness.  How marvelous are Your works, O Lord!  Thank You.  Father, I ask that You would shine this glorious Light, the Truth of Your Word, into our hearts and into the hearts of my loved ones. I pray for their salvation and for their sanctification through Your Word and through the Living Water, Your Holy Spirit.  It is in Christ's Name that I ask this . amen.