A.T. Robertson Commentary 1 Corinthians 15:10

"But by the grace of God I am what I am: and his grace which was bestowed upon me was not found vain; but I labored more abundantly than they all: yet not I, but the grace of God which was with me." — 1 Corinthians 15:10 (ASV)

What I am (ο ειμ). Not,

who (ος), but

what (ο), neuter singular. His actual character and attainments. All "by the grace of God" (χαριτ θεου).

I laboured more abundantly than they all (περισσοτερον αυτων παντων εκοπιασα). This is sober fact as shown by the Acts and Paul's Epistles. He had tremendous energy and used it. Genius is work, Carlyle said. Take Paul as a specimen.