@latein @ArdainianRight @hachi @djsumdog @meowski @BowsacNoodle @mrsaturday If you want a definition of what a true Christian believes then look at the Apostle's Creed, the Nicene Creed, and the Athanasian Creed:
https://bookofconcord.org/ecumenical-creeds/
Gnosticism, no matter the flavour, is not Christian. Believing that the God of the Old Testament is the Demiurge is not Christian.
Christians confess that there is only one God who is the sole creator of all things which leaves no space for a Demiurge, Christians also confess that God is good which leaves no space for a Demiurge, and Christians confess that God created all things out of nothing which leaves no space for a Demiurge.
The idea that the God of the Old Testament is the Demiurge is absurd and only came about because people couldn't reconcile God punishing people in the OT and dying on the cross for them in the New Testament, completely missing all the ways in which God demonstrates His mercy and compassion in the OT. It's the same God and the OT and NT link together in so many ways that there is no room for the God of the OT to be a Demiurge.
As for Jesus being a mortal man possessed by "the Christ" or the Holy Spirit. Christians confess that Jesus is God, not that He is of a similar nature but that He is God. Read the Nicene Creed "very God of very God" and "being of one substance with the Father" spell it out pretty clearly. This leaves no room for "Jesus was a mortal man possessed by the Christ" in actual true Christian belief. If Jesus was just a skinsuit being worn by the Holy Spirit or something then the entire foundation of the faith is nothing but sand and Christians are to be pitied because a skinsuit cannot save.
Have you ever actually looked at what Christians believe and compared it to what gnostics believe? Seriously, have a listen to a couple of @SuperLutheran 's episodes in the playlist I linked, it's night and day.
As for "well if someone calls themselves Christian then they must be!":
21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’ (Matthew 7:21-23 ESV)
This is why a Christian is judged to be true or not based on whether they bear fruit, as in do they live according to God's will? Do they love their fellow Christian? Do they help people in need? The Devil can appear as an angel of light, wolves can wear sheep's clothing, anyone can call themselves a Christian but that doesn't mean they are.