What does it mean that grace was not in vain?

When Paul states that God's grace was not in vain, he is highlighting that the grace he received led to tangible actions and results in his ministry. In 1 Corinthians 15:10, he explains that although he labored more abundantly than others, it was not his effort alone but the grace of God working through him that enabled this labor. This teaches us that true grace is transformative; it should lead to a life marked by obedience and service in response to God's kindness. The evidence of grace in our lives is seen through how we live out our faith and respond to God’s calling to share the Gospel with others.