Tertullian, Against Marcion 4.38
Christ knew where John's baptism came from, so why did He ask as if He did not? He knew the Pharisees would not answer Him, so why ask in vain? Was it so that He might judge them from their own mouths, or from their hearts? Refer these points, then, to the defense of the Creator and to the comparison with Christ, and consider now what would have followed if the Pharisees had given some reply to the question. Suppose they had replied that John's baptism was human: they would have been stoned immediately. Some Marcion would have arisen against Marcion to say: 'Oh best God, oh God different from the Creator's examples!' Knowing that men would rush headlong, He Himself put them on the precipice. For thus the Creator is also treated in the law of the tree. 'But John's baptism was from heaven.' And why, says Christ, did you not believe him? Therefore, the one who was going to rebuke them for not believing was the one whose sacrament John was administering.
Latin
Sciebat Christus, baptisma Iohannis unde esset. et cur quasi nesciens interrogabat? sciebat non responsuros sibi Pharisaeos. et cur frustra interrogabat? an. ut ex ore ipsorum iudicaret illos uel ex corde? refer ergo et haec ad excusati. onem creatoris et ad comparationem Christi, et considera iam, quid secuturum esset, si quid Pharisaei ad interrogationem renuntiassent. puta illos renuntiasse humanum Iohannis baptisma: statim lapidibus elisi- fuissent. existeret aliqui Marcion aduersus Marcionem, qui diceret: o deum optimum, o deum diuersum a creatoris exemplis! sciens praeceps ituros homines ipse illos in praerupium imposuit. sic enim et de creatore in arboris lege tractatur. 'sed de caelis fuit baptisma Iohannis). et quare, inquit Christus, non credidistis ei? ergo qui credi uoluerat Iohanni, increpaturus quod non credidissent, eius erat, cuius sacramentum Iohannes administrabat.