John Chrysostom, on 1Cor 15:5

"And that He appeared to Cephas:" he names immediately the most credible of all. "Then to the twelve."

Thus, since he had mentioned the proof from the Scriptures, he adds also that by the events, producing as witnesses of the resurrection, after the prophets, the apostles and other faithful men. Whereas if he meant that other resurrection, the deliverance from sin, it were idle for him to say, He appeared to such and such an one; for this is the argument of one who is establishing the resurrection of the body, not of one obscurely teaching deliverance from sins. Wherefore neither said he once for all, "He appeared," although it were sufficient for him to do so, setting down the expression in common: but now both twice and thrice, and almost in each several case of them that had seen Him he employs it. For "He appeared," saith he, "to Cephas, He appeared to above five hundred brethren, He appeared to me also." Yet surely the Gospel saith the contrary, that He was seen of Mary first. But among men He was seen of him first who did most of all long to see Him.

But of what twelve apostles doth he here speak? For after He was received up, Matthias was taken into the number, not after the resurrection immediately. But it is likely that He appeared even after He was received up. At any rate, this our apostle himself after His ascension was both called, and saw Him. Therefore neither doth he set down the time, but simply and without defining recounts the appearance. For indeed it is probable that many took place; wherefore also John said, "This third time He was manifested."

Therefore he appears to Peter first. For he that first confessed Him to be Christ was justly also counted worthy first to behold His resurrection. And not on this account alone doth He appear to him first, but also because he had denied Him, more abundantly to comfort him and to signify that he is not despaired of, before the rest He vouchsafed him even this sight and to him first entrusted His sheep. Therefore also He appeared to the women first. Because this sex was made inferior, therefore both in His birth and in His resurrection this first tastes of His grace.