Note 1 at 2 Cor. 6:1: Paul is now continuing his thought that we are ambassadors for Christ (2 Cor. 5:20), working together with God (2 Cor. 6:1). In the Greek text "workers together" is one word, "sunergeo," and is used in the New Testament a total of five times (Mk. 16:20; Rom. 8:28; 1 Cor. 16:16; 2 Cor. 6:1; Jas. 2:22). It carries the idea of something or someone that is a partner in labour, assisting, and helping with work. In this verse it is used as a verb in the active voice. It's something we are doing together with God.

Note 2 at 2 Cor. 6:1: Paul encourages the Corinthians not to receive the grace of God in vain. The word "vain" carries the idea of something that does not yield the desired outcome; something that is without effect or avail. God's grace alone doesn't save us (see note at Eph. 2:8). To have the desired effect, we have to mix what God has done for us with faith (Heb. 4:2).

There are many scriptures that refer to the grace of God being without effect in our life. Here are a few of those references: "...If righteousness come by the law, then Christ is dead in vain." (Gal. 2:21); "If Christ be not risen, then is our preaching vain, and your faith is also vain." (1 Cor. 15:14); "In vain they do worship me, teaching for doctrines the commandments of men." (Mt. 15:9); "If any man among you seem to be religious, and bridleth not his tongue,...this man's religion is vain." (Jas. 1:26); "...lest by some means the tempter have tempted you, and our labour be in vain." (1 Th. 3:5); "I am afraid of you, lest I have bestowed upon you labour in vain." (Gal. 4:11); "...I have not run in vain, neither laboured in vain." (Phil. 2:16); "...forasmuch as ye know that your labour is not in vain in the Lord." (1 Cor. 15:58); "For yourselves, brethren, know our entrance in unto you, that it was not in vain" (1 Th. 2:1).