Cerdo (Greek: Κέρδων) was a Syrian gnostic who was deemed a heretic by the Orthodox Church around the time of his teaching, circa 138 AD. Cerdo started out as a follower of Simon Magus. He taught at about the same time as Valentinus and preceded Marcion. According to Irenaeus, he was a contemporary of the Roman bishop Hyginus, residing in Rome as a prominent member of the Church until his forced expulsion therefrom.
He taught that there were two gods, one that demanded obedience while the other was good and merciful. According to Cerdo, the former was the God of the Old Testament who had created the world. He also said that the latter God was superior but that he was only known through his son, Jesus. Like later gnostics, he was a docetist who rejected the bodily resurrection of the dead.
According to the account of Irenaeus ( and ), Cerdo had not the intention of founding a sect apart from the church. He describes him as more than once coming to the church and making public confession, and so going on, now teaching his doctrine in secret, now again making public confession, now convicted in respect of his evil teaching, and removed from the communion of the brethren (aphistamenos tes ton adelphon synodias). Some understand this to mean that Cerdo voluntarily withdrew himself from communion, but it is preferred to understand the word passively, with the old translator of Irenaeus, "abstentus est a religiosorum hominum conventu."
The account given by Irenaeus of the doctrine of Cerdo is that he taught that the God preached by the law and the prophets was not the Father of Jesus Christ; for that the former was known, the latter unknown; the former was just, the latter good. The account given by Pseudo-Tertullian () may be regarded as representing that given in the earlier Syntagma of Hippolytus, which was also made use of by Philaster (Haer. 44) and Epiphanius (Haer. 41). It runs that Cerdo introduced two first principles (archai) and two gods, the one good, the other evil, the latter of whom was the creator of the world. Here we are to note the important difference that to the good god is opposed in the account of Irenaeus a just one; in that of Hippolytus, an evil one. of Hippolytus, Cerdo is said to have taught three principles of the universe, , dikaion, hylen. Ps.-Tertullian goes on to say that Cerdo rejected the law and the prophets, and renounced the Creator, teaching that Christ was the son of the higher good deity, and that he came not in the substance of flesh but in appearance only, and had not really died or really been born of a virgin. He adds that Cerdo only acknowledged a resurrection of the soul, denying that of the body. Ps.-Tertullian adds, but without support from the other authorities, that Cerdo received only the Gospel of St. Luke, and that in a mutilated form; that he rejected some of Paul's epistles and some portions of others, and completely rejected the Acts and the Apocalypse. There is every appearance that Ps.-Tertullian here transferred to Cerdo what in his authority was stated of Marcion.