Yet he did not waver through disbelief in the promise of God, but was strengthened in his faith and gave glory to God, — Romans 4:20

Paul knew who he was — a former persecutor of the church, the chief of sinners. But he also knew who he had become: by the grace of God I am what I am. He didn't minimize his labor for the Gospel, but he traced it all back to grace. His toil wasn't in vain, not because he was so diligent, but because God's grace was with him. Whatever you are in Christ, it's grace that made you that way — and grace that keeps you.