Note here that Paul is saying he is less than the least of all saints because he is deeply grateful for the grace of God. He knows that without grace, his religious righteousness, which looked to be perfect to those around him, was worthless and had him just as hell bound as any other murderer.

Paul said some very confusing things in this passage. Things like:
“If somehow I may attain to the resurrection”
“Not that I have already grasped it all or become perfect”
“I do not regard myself as having taken hold of it yet”