1 Corinthians 15:2

15:2 and by which you are being saved, if you hold firmly to the message I preached to you – unless you believed in vain.

1 Corinthians 15:10

15:10 But by the grace of God I am what I am, and his grace to me has not been in vain. In fact, I worked harder than all of them – yet not I, but the grace of God with me.

1 Corinthians 15:58

15:58 So then, dear brothers and sisters, be firm. Do not be moved! Always be outstanding in the work of the Lord, knowing that your labor is not in vain in the Lord.