A.T. Robertson Commentary 1 Corinthians 15:9

"For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God." — 1 Corinthians 15:9 (ASV)

The least (ο ελαχιστος). True superlative, not elative. Explanation of the strong word εκτρωμα just used. See Eph 3:8 where he calls himself "less than the least of all saints" and 1 Timothy 1:15 the "chief" (πρωτος) of sinners. Yet under attack from the Judaizers Paul stood up for his rank as equal to any apostle (2 Corinthians 11:5f.,23).

Because I persecuted the church of God (εδιωξα την εκκλησιαν του θεου). There were times when this terrible fact confronted Paul like a nightmare. Who does not understand this mood of contrition?