7 Such, then, are the first principles of the Gospel: that there is one God, the Maker of this universe; He who was also announced by the prophets, and who by Moses set forth the dispensation of the law,— [principles] which proclaim the Father of our Lord Jesus Christ, and ignore any other God or Father except Him. So firm is the ground upon which these Gospels rest, that the very heretics themselves bear witness to them, and, starting from these [documents], each one of them endeavours to establish his own peculiar doctrine.
For the Ebionites, who use Matthew's Gospel only, are confuted out of this very same, making false suppositions with regard to the Lord. But Marcion, mutilating that according to Luke, is proved to be a blasphemer of the only existing God, from those [passages] which he still retains. Those, again, who separate Jesus from Christ, alleging that Christ remained impassible, but that it was Jesus who suffered, preferring the Gospel by Mark, if they read it with a love of truth, may have their errors rectified.
Those, moreover, who follow Valentinus, making copious use of that according to John, to illustrate their conjunctions, shall be proved to be totally in error by means of this very Gospel, as I have shown in the first book. Since, then, our opponents do bear testimony to us, and make use of these [documents], our proof derived from them is firm and true.
Source: Adversus haereses (New Advent)