11 Corinthians 6:1 & 7:1

6:1 We appeal to you not to receive the grace of God in vain.
7:1 Since we have these promises, beloved, let us cleanse ourselves from every defilement of body and spirit, bringing holiness to completion in the fear of God.

To be his people (6:16,17) means to be walking with Jesus (16), and cleansed from sin. Letting him completely transform me into the child of his that I am . This is how his grace is not in vain.