1 Corinthians 15:8

And last of all he was seen of me also, as of one born out of due time (KJV)

Then: Paul saw Christ after the ascension had occured. That's what he means by "out of due time." As far as the gospel record is concerned, everyone else saw him before the ascension. All the disciples and many of the other apostles had walked and talked with Jesus. Not him. But when Paul himself saw the risen Lord, that changed everything in his life. He was the last and he didn't feel worthy (Henry).

Now: We read of Paul's works and if we take the time, never expect to do the things Paul did. We depend on knowing our salvation through the Lord's presence in our lives, his power in directing and controlling it, and, at the point of need, the spiritual gifts he gives us. We take it as a fact that Paul was an apostle.

Extra Source: Commentary on the Whole Bible Acts to Revelation Vol VI by Matthew Henry online at http://www.ccel.org/ccel/henry/mhc6.iCor.xvi.html