8 Elsewhere Faustus says: When you find no passage to point to, you use this weak and inappropriate argument, that a Christian is bound to believe Christ when he says that Moses wrote of Him, and that whoever does not believe this is not a Christian. It would be far better to confess at once that you cannot find any passage. This argument might be used with me, because my reverence for Christ compels me to believe what He says. Still it may be a question whether this is Christ's own declaration, requiring absolute belief, or only the writer's, to be carefully examined.
And disbelief in falsehood is no offense to Christ, but to impostors. But of whatever use this argument may be with Christians, it is wholly inapplicable in the case of the Jew or Gentile, with whom we are supposed to be discussing. And even with Christians the argument is objectionable. When the Apostle Thomas was in doubt, Christ did not spurn him from Him. Instead of saying, "Believe, if you are a disciple; whoever does not believe is not a disciple," Christ sought to heal the wounds of his mind by showing him the marks of the wounds in His own body.
Does it become you then to tell me that I am not a Christian because I am in doubt, not about Christ, but about the genuineness of a remark attributed to Christ? But, you say, He calls those especially blessed, who have not seen, and yet have believed. If you think that this refers to believing without the use of judgment and reason, you are welcome to this blind blessedness. I shall be content with rational blessedness.
Source: Reply to Faustus the Manichaean (New Advent)