A.T. Robertson Commentary 2 Corinthians 6:1

"And working together [with him] we entreat also that ye receive not the grace of God in vain" — 2 Corinthians 6:1 (ASV)

Working together with him (συνεργουντες). We are co-workers, partners with God (1 Corinthians 3:9), in this work of grace.

In vain (εις κενον). Into emptiness. The plan of God, the work of Christ on the Cross, the pleas of the ambassador may all be nullified by the recipient of the message.