If the Gnostic Gospels, such as the Coptic Gospel of [pseudo-]Thomas, are supposedly as valuable for understanding the life of Jesus as the canonical New Testament, how come Tatian made no use of them?
During the middle of the 2d century, an Assyrian by the name of Tatian, or Addai, became a Christian. He was under the tutelage of Justin Martyr, but when Justin died, Tatian became a follower of the Gnostic Valentinus. He was then excommunicated from the Christian church in Rome, and moved eastwards first to Macedonia, then later to his native Mesopotamia. Scholars agree that he died around 185 A.D.
Tatian's main claim to fame was the production of the Diatessaron ("through the four"), which synthezied the four Gospels into a single, continuous narrative. This apparently remained "the' Gospel for Syriac-speaking Christians until roughly a century later, when it was supplanted by the Pehsitto's separated Gospels.
Despite adhering to a system of doctrine closer to that of Valentinus than to that of the orthodox, Tatian made no use of the Gnostic Gospels in his Diatessaron. This would be strange given both his own Gnostic predilections had the Gnostic material enjoyed wide currency and status as a legitimate "alternative" Christianity in Tatian's time. Given that there would be no state-sanctioned persecution of Christian heresy until roughly a century and a half after Tatian's death, such an omission may require an explanation.
Quite simply, the Gnostic gospels did not exist in Tatian's day,during the second third of the second century. He was probably instructed in Gnostic teachings through treatises or word of mouth. Perhaps the Gnostic gospels were composed as Gnostic adherents recognized that the Jesus of the four canonical Gospels was not "one of us". Perhaps Tatian's Diatessaron served not only the purposes of orthodox believer, but inadvertently drove home to Tatian's fellow Gnostics the wide gulf between their beliefs and those of the historical Jesus.