They say Jesus was not an actual man, only had the appearance of a man. They say all matter is evil.
1Jn 4:3 And every spirit that confessed not that Jesus Christ is come in the flesh is not of God: and this is that spirit of Antichrist, whereof ye have heard that it should come; and even now already is it in the world.
2Jn 1:7 For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an Antichrist.
When Christianity first started, some aspects of the faith the faith were more esoteric than today. The Gnostics believed that the whole universe was created by an "evil" god who was opposed to the non-material "good" god. They saw the "creator god" of the Old Testament as the "bad guy." The "real" Jesus was only Jesus's actual soul, which could only be freed by killing his body. Some people think the Gospel of Judas was a gnostic text as Judas claimed to have gotten a special teaching from Jesus which the other apostles did not get. Jesus asked Judas to arrange the killing of his "body" so the "real Jesus" could be freed. The Gnostics and the other early christian church must have competed for converts on some level but the "standard" version of Christianity won out.
Gnosticism is based on the idea that there is "secret knowledge" (nosies) which Jesus passed on to some or all of the apostles or favored disciples, like Mary Magdalene. This knowledge would permit people's spirits to escape the trap of the material world. The emphasis on "faith" was for the outer church members who weren't ready for the secret and possibly never would be, but the elect would be taken into the inner circle and taught the secrets.
You can see why the leadership of the "outer" church - much the more numerous - didn't particularly like this doctrine and stamped it out wherever they found it. It kept popping up, though, and still does in one form or another.
Their main draw for me was the free thinking search for God.
The belief that God has access to all souls directly. No need for a huge Church pyramid of rank, is also more in line with what God would build. In terms of a demographic pyramid, that is.
God in fact, speaking through men has always been the way of it.
They did support the idea of local Churches.
A true Gnostic Fundamental of the old stripe would then be one who has felt the voice of the God within and tries to interpret Christian scriptures in a way that is against literal interpretations. Many forget that they treat their own scriptures the same way. They are to be a focus for thought towards God. Not a literal works.
A true Gnostic will lean towards a naturalist inclination but have an itch, shall we say, that there is more out there than meets the eye.
In the spirit, is a term used by those who professed to have spoken to God. I call this telepathic communication.
I did this one time only and became a born Gnostic Christian, so to speak.