What is Christian Gnosticism? And how do we approach that subject when it comes to sceptical indecduals who question Christianity?
The type of Gnosticism preached and practiced has changed over time. But the basis of Gnosticism is that a person can be liberated through special knowledge (Gnosis). It was a huge problem in the early church but I do not think that the problems of Gnosticism is only in the past. It was very appealing. Even St. Augustine was a devout Manichaen (A type of Gnostic cult) before he came to Christianity.
One of the main Christological heresies of Gnosticism was that they denied the humanity of Jesus. They said that Jesus only appeared to be human and came to give human beings special knowledge through which they could be liberated. They also believed that the world is evil and all material existence is evil. Only the true spirit is good and hence Jesus could not actually have a material body.
Instead of sin being the basic problem of humanity, Gnostics claim that ignorance is the basic problem and that problem can only be cured through enlightenment. Gnosticism does have similarities with many easter philosophies especially Advaita Vedanta (Hinduism) and Buddhism.
I am attaching a link here which can help you look further into the topic. But the points mentioned above are the main tenets of Gnosticism. One way by which we approach this question when people ask is to acknowledge the fact that Christianity had to battle many issues during the early centuries and it did battle it victoriously. Gnosticism was definitely a huge internal threat and one that the early Christians dealt with wisely.
Gnosticism and the Early Church