1 Corinthians 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. […]

After these points, Celsus proceeds to bring against the Gospel narrative a charge which is not to be lightly passed over, saying that "if Jesus desired to show that his power was really divine, he ought to have appeared to those who had ill-treated him, and to him who had condemned him, and to all men universally." For it appears to us also to be true, according to the Gospel account, that He was not seen after His resurrection in the same manner as He used formerly to show Himself — publicly, and to all men. But it is recorded in the Acts, that "being seen during forty days," He expounded to His disciples "the things pertaining to the kingdom of God." [Acts 1:3] And in the Gospels it is not stated that He was always with them; but that on one occasion He appeared in their midst, after eight days, when the doors were shut [John 20:26], and on another in some similar fashion. And Paul also, in the concluding portions of the first Epistle to the Corinthians, in reference to His not having publicly appeared as He did in the period before He suffered, writes as follows: "For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the Scriptures; and that He was seen of Cephas, then of the twelve: after that He was seen of above five hundred brethren at once, of whom the greater part remain unto the present time, but some are fallen asleep. After that He was seen of James, then of all the apostles. […]

Note that he does not say "to the eleven," and neither does John (Jn 20:24), who writes that Thomas was "one of the twelve." We should probably say that either he has included Matthias with the other apostles by anticipation or else that he is …

First he puts forward the most trustworthy witness of all. Although the Gospel says that the Lord appeared first to Mary (Mark 16:9), yet among men He appeared first to Peter, as the chief of the disciples. For it was fitting that he who first …

898. – I delivered to you, fourthly, the appearance of Christ, because he appeared to Cephas. And he presents first the appearance made to others; secondly, those make to himself alone (v. 8). 899. – In should be noted, however, in regard to the first, …