Marcion's rejection of the unity and coherence of Old and New Testaments entailed abandonment of the contention that ancient prophecies were fulfilled in Christ and his Church. That contention was important in the conversion of Justin. He was born to Gentile parents in Samaritan territory at Nablus (Neapolis), and travelled in search of philosophical education. He claimed to have sat at the feet of a Stoic, a Peripatetic who disappointed him by concern for his fee (this was a cliché about Aristotelian tutors), and a Pythagorean who expected him to have mastered arithmetic, geometry, music, and astronomy so as to grasp immaterial truths. It was a search for truth in the soul, and of the various schools of philosophy the Platonists had most to offer by way of a religious quest. Plato's Phaedrus held up the goal of a celestial vision of God. But Justin was persuaded to abandon Platonism by a seashore conversation with an old man who told him about the Hebrew prophets and the fulfilment of their predictions in Jesus' virgin birth, incarnation, passion, descent to Hades, ascension and 'Son of God' title. Since a substantial part of New Testament Christology was formulated in terms derived from the Old Testament, the argument had force. The old man also deployed Aristotle's arguments against Platonism.
Justin remained positive towards Platonism, 'not radically different from Christianity but not quite the same' (Apol. II 13). He believed the thesis of earlier Jewish argument that Plato had studied the writings of Moses, especially Genesis 1, in composing his Timaeus. Moreover the Platonic theodicy which attributed responsibility for evil to free choices by rational beings Justin welcomed as derived from Moses (Apol. I 44). Greek philosophers derived from the prophets their true ideas of the soul's immortality and judgement hereafter, A foreshadowing of the insight that in God there is a threeness is evident in Plato's second letter. In classical philosophical schools there are 'seeds of truth, sent down to humanity' (Dial. 2. 1; Apol. II 8), but not in the hedonism of Epicurus. Socrates, Heraclitus, and the Stoic Musonius Rufus exiled under Nero were martyrs for truth. Jesus's parable of the sower could be applied to the seeds of philosophical truth scattered along the wayside by providence.
Of the superiority of Christ's ethical teaching Justin was confident,