And be not surprised if all the multitudes who have believed on Jesus do not behold His resurrection, when Paul, writing to the Corinthians, can say to them, as being incapable of receiving greater matters, “For I determined not to know anything among you, save Jesus Christ, and Him crucified;” which is the same as saying, “Hitherto you were not able, neither yet now are you able, for you are still carnal.” The Scripture, therefore, doing everything by appointment of God, has recorded of Jesus, that before His sufferings He appeared to all indifferently, but not always; while after His sufferings He no longer appeared to all in the same way, but with a certain discrimination which measured out to each his due. And as it is related that “God appeared to Abraham,” or to one of the saints, and this “appearance” was not a thing of constant occurrence, but took place at intervals, and not to all, so understand that the Son of God appeared in the one case on the same principle that God appeared to the latter.
Source: Against Celsus (New Advent)