33 You see, my argument is not that if you are a Christian you must believe Christ when He says that Moses wrote of Him, and that if you do not believe this you are no Christian. The account you give of yourself in asking to be dealt with as a Jew or a Gentile is your own affair. My endeavor is to leave no avenue of error open to you. I have shut you out, too, from that precipice to which you rush as a last resort, when you say that these are spurious passages in the Gospel; so that, freed from the pernicious influence of this opinion, you may be reduced to the necessity of believing in Christ.
You say you wish to be taught like the Christian Thomas, whom Christ did not spurn from Him because he doubted of Him, but, in order to heal the wounds of his mind, showed him the marks of the wounds in His own body. These are your own words. It is well that you desire to be taught as Thomas was. I feared you would make out this passage too to be spurious. Believe, then, the marks of Christ's wounds. For if the marks were real, the wounds must have been real. And the wounds could not have been real, unless His body had been capable of real wounds; which upsets at once the whole error of the Manichæans.
If you say that the marks were unreal which Christ showed to His doubting disciple, it follows that He must be a deceitful teacher, and that you wish to be deceived in being taught by Him. But as no one wishes to be deceived, while many wish to deceive, it is probable that you would rather imitate the teaching which you ascribe to Christ than the learning you ascribe to Thomas. If, then, you believe that Christ deceived a doubting inquirer by false marks of wounds, you must yourself be regarded, not as a safe teacher, but as a dangerous impostor.
On the other hand, if Thomas touched the real marks of Christ's wounds, you must confess that Christ had a real body. So, if you believe as Thomas did, you are no more a Manichæan. If you do not believe even with Thomas, you must be left to your infidelity.
Source: Reply to Faustus the Manichaean (New Advent)