And since Celsus has termed the apostles of Jesus men of infamous notoriety, saying that they were tax-gatherers and sailors of the vilest character, we have to remark, with respect to this charge, that he seems, in order to bring an accusation against Christianity, to believe the Gospel accounts only where he pleases, and to express his disbelief of them, in order that he may not be forced to admit the manifestations of Divinity related in these same books; whereas one who sees the spirit of truth by which the writers are influenced, ought, from their narration of things of inferior importance, to believe also the account of divine things.
Now in the general Epistle of Barnabas, from which perhaps Celsus took the statement that the apostles were notoriously wicked men, it is recorded that “Jesus selected His own apostles, as persons who were more guilty of sin than all other evildoers.” And in the Gospel according to Luke, Peter says to Jesus, “Depart from me, O Lord, for I am a sinful man.” Moreover, Paul, who himself also at a later time became an apostle of Jesus, says in his Epistle to Timothy, “This is a faithful saying, that Jesus Christ came into the world to save sinners, of whom I am the chief.” And I do not know how Celsus should have forgotten or not have thought of saying something about Paul, the founder, after Jesus, of the Churches that are in Christ.
He saw, probably, that anything he might say about that apostle would require to be explained, in consistency with the fact that, after being a persecutor of the Church of God, and a bitter opponent of believers, who went so far even as to deliver over the disciples of Jesus to death, so great a change afterwards passed over him, that he preached the Gospel of Jesus from Jerusalem round about to Illyricum, and was ambitious to carry the glad tidings where he needed not to build upon another man's foundation, but to places where the Gospel of God in Christ had not been proclaimed at all.
What absurdity, therefore, is there, if Jesus, desiring to manifest to the human race the power which He possesses to heal souls, should have selected notorious and wicked men, and should have raised them to such a degree of moral excellence, that they became a pattern of the purest virtue to all who were converted by their instrumentality to the Gospel of Christ?
Source: Against Celsus (New Advent)