2 John is a letter written to a specific Christian community by “the Elder.” Earliest tradition attributes the letter to the same hand as 1 John and was written from Ephesus. 2 John also addresses the Gnostic heresy that denies the incarnation (that God became flesh and lived among us.) Key Learning: Jesus is God incarnate come to live among us.
The key argument in 2 John is verse 9: "Everyone who does not abide in the teaching of Christ, but goes beyond it, does not have God;" The "goes beyond it is the key. The Gnostics had been teaching that the Christian gospel was a "jumping off" point, merely the beginning of the special knowledge needed for salvation (the rest of the special knowledge was in the apparently in the possession of the Gnostics). The early Christians believed that the life, death and resurrection of Jesus along with the coming of the Holy Spirit was the complete story necessary for salvation. It should not be reduced nor augmented.