For I am the least of all the apostles, unworthy to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am, and his grace towards me was not in vain.
"I am what I am" is a popular phrase used to excuse almost anything. Yes, I know I gossip/treat my spouse badly/react angrily to criticism/don't pray/don't read my bible/don't go to church if something better comes up... but that's just me, I am what I am! Paul tells the Corinthian church that "by the grace of God I am what I am". Is this justification for our self justification? God is gracious so who cares that I'm not?
That is the opposite of Paul's meaning. He is astounded by God's grace that has taken him from persecuting the church and, as unworthy as he is, has made him an apostle. God's grace transformed Paul from someone trying to kill off the church to someone who would spend the rest of his life planting churches! Paul's cry of "I am what I am" is "can you believe God is so gracious that I am what I am?" not "don't expect me to change".
God's grace towards Paul was not in vain. What about us? It changed him. Is it changing us?