1 Corinthians 15:8

Paul regarded the Lord’s appearance to him on the Damascus road as an equivalent post-resurrection appearance and the Lord’s last one.

"Paul thinks of himself here as an Israelite whose time to be born again had not come nationally (cp. Matthew 23:39), so that his conversion by the appearing of the Lord in glory (Acts 9:3-6) was an illustration, or instance, before the time of the future national conversion of Israel. See Ezekiel 20:35-38; Hosea 2:14-17; Zechariah 12:10 to Zechariah 13:6; Romans 11:25-27; 1 Timothy 1:16)." [Note: The New Scofield . . ., p. 1247.]

Another better view, I think, is that Paul meant that he had become an apostle after the Twelve had become apostles.

Paul may have referred to himself as he did (lit. an abortion) not because his apostleship came to him prematurely. The Lord appointed him some time after the others. He may have done so because compared with the backgrounds and appointments of the other apostles Paul’s were unusual. He lacked the normal "gestation period" of having accompanied the Lord during His earthly ministry (cf. Acts 1:21-22).

"Since this is such an unusual term of deprecation, and since it occurs with the article, the ’abortion,’ it has often been suggested that the Corinthians themselves have used the term to describe Paul, as one who because of his personal weaknesses is something of a ’freak’ in comparison with other apostles, especially Apollos and Peter. Others have suggested that the term is a play on Paul’s name-Paulus, ’the little one.’ Hence they dismissed him as a ’dwarf.’ This has the advantage of helping to explain the unusual ’digression’ in 1 Corinthians 15:9-10, where he in fact allows that he is ’least’ of all the apostles; nonetheless God’s grace worked the more abundantly in his behalf.

"In any case, whether it originated with them, which seems altogether likely, or with Paul himself in a sudden outburst of self-disparagement, it seems hardly possible to understand this usage except as a term that describes him vis-à-vis the Corinthians’ own view of apostleship." [Note: Fee, The First . . ., p. 733.]

Paul stressed the appearances of the risen Christ (1 Corinthians 15:5-9) because they prove that His resurrection was not to a form of "spiritual" (i.e., non-corporeal, not physical or material) existence. Just as His body died and was buried, so it was raised and many witnesses saw it, often many witnesses at one time.