When Paul talks about grace he could be talking about the grace of salvation offered to us through faith in Jesus. But, Second Corinthians 6:1 Paul is referring to the grace each believer receives in order to serve God. The context in Second Corinthians to "receive God's grace" is in reference to the gifting of believers just as Paul spoke of in Romans 12:3-6 where he says:
"By the grace given me...we have different gifts according to the grace given us."
- Romans 12:3, 6
Peter speaks of this grace of being gifted for service in First Peter 4:10-11:
"Each of you should use whatever gift you have received to serve others, as faithful stewards of God’s grace in its various forms." - 1 Peter 4:10
In Second Corinthians 6 Paul quotes from Isaiah 49:8 where God says he will hear "in the time of my favor" and God will help "in the day of salvation." Isaiah writes of waiting for God's "time of favor" and God's "day of salvation." The impression is great things will happen when it is "time" and when that great "day of salvation" comes. Isaiah spoke of a time when God would hear prayers and would respond in supernatural ways. Isaiah foresaw a day when God would bring deliverance and provide salvation for people who were seeking his help.
Paul uses Isaiah's verse to set the stage to encourage the Corinthians "not to receive God's grace in vain," or, as Peter calls it, "God's grace in its various forms." The Corinthians had been gifted to work as God directed because "the time of God's favor" and "the day of salvation" that Isaiah spoke of was "now" according to Paul. Paul did not want the Corinthians to miss this day of salvation and hour of power. Through the Holy Spirit empowered people called the church God was to bringing salvation to a generation in 56 AD.
To fail to understand this or to continue to wait for God was to squander the grace that the Old Testament prophets could only say was coming sometime in the future. We are living in the "time of God's favor" and in "the day of salvation." As believers we have been empowered to bring our gifts to our generation. Do not receive your gift in vain! Now is your time to serve God in power according to the gift you have received.
"As God's co-workers we urge you not to receive God's grace in vain...I tell you, now is the time...now is the day!" - Second Corinthians 6:1-2