After this he says, that certain of the Christian believers, like persons who in a fit of drunkenness lay violent hands upon themselves, have corrupted the Gospel from its original integrity, to a threefold, and fourfold, and many-fold degree, and have remodelled it, so that they might be able to answer objections. Now I know of no others who have altered the Gospel, save the followers of Marcion, and those of Valentinus, and, I think, also those of Lucian. But such an allegation is no charge against the Christian system, but against those who dared so to trifle with the Gospels. And as it is no ground of accusation against philosophy, that there exist Sophists, or Epicureans, or Peripatetics, or any others, whoever they may be, who hold false opinions; so neither is it against genuine Christianity that there are some who corrupt the Gospel histories, and who introduce heresies opposed to the meaning of the doctrine of Jesus.
Source: Against Celsus (New Advent)