The Gnostics believed they had access to the truth; the real truth, not the truth that everyone else thinks is the truth. Everyone who's not a Gnostic that is. While there are certain themes that tend to unite Gnostic groups, they were actually quite distinct and widespread across the Christian world in the time of the Church Fathers. Christian Gnostics—who will be our focus although Hermeticists are also sometimes classified as Pagan Gnostics—tended to believe that the Old Testament God or Yahweh was actually a demigod and that the true God was unknowable, existing in an unimaginable realm somewhere in the cosmological beyond. They tended to believe that humans possessed some grain or seed of the godhead within them and they often underwent elaborate astrologically-themed initiations to join their orders. While their particular theology was ultimately defeated and buried by the Catholic Christians, their beliefs informed Christian doctrine. Arguably, the canonical gospel of John was, in fact, a Gnostic text and a Gnostic bishop very nearly became the Pope in Rome. But Gnostics were considered heretics and the men who defined early Christian doctrine wrote bitter attacks against them. Ironically, these attacks became a significant source for contemporary scholars' knowledge of the ancient Gnostics beliefs and practices. Be careful how detailed you are in arguing against your enemies. You may just be preserving their ideas across the ages.