NOTES for Co1 15:10
Apostle Paul evaluates his service and himself very soberly; he understands very well that his gift of apostolate is a gift of the grace of God, God's will. From the jealous persecutor of Christians, he becomes one of the most diligent apostles of Christ after Jesus appeared to him on the road leading to Damascus.
God shows the initiative, he calls man and grants him grace for the service. But several things depend on man: there can be no results; the grace can turn out vain, if we do not put all our efforts on the work of God.
Even if all efforts are applied, it is important to understand, as the apostle sees it so clear, that I do not work by my own strengths, but by "the grace of God which is with me".