@Pepperwort -well I suppose it would depend on your definition of Christianity and the definition of God and Christ. If you limited the definition to that given by the rules of the Latin church then yes it has been deemed heresy. However, if you took the broader approach a Christian could be anyone who follows Christ.
Therefore one would therefore have to examine the concept of Christ. Nowadays we think of Christ as a man, the flesh and blood son of God. But many of what we now call the Gnostics believed Christ was a spiritual concept. The Gospels Anthropomorphised the spiritual essence to make stories which were easy to relate to and pass on through oral tradition.
If Christ isn’t a person When you look at the Gospels what is the point of the stories? My own personal view is they are stories to embrace and encourage change. The first miracle (or indeed alchemical success) is the transmutation of water into wine, we see death being overcome, mastery of the elements (walking on water). The slaughter of innocents by Herod matched the slaughter of the first born by pharaoh. It was a call to arms, to change the status quo, to transmute slavery to freedom. A new covenant, a new type of God (following gnostic duslism)Obviously the criticism of the Romans is most noticeable in Johns Revelations but it runs throughout the Bible. The call to change, the reward for change as a central theme to the rest is hard to ignore once noticed. Christ was effectively what we would later recognise as a spiritual philosophers stone. The flesh and blood Christ (if he did exist) was most likely a man like Brian (although I have no idea if he was a very naughty boy😂).
If we take Christ as a spiritual concept then there would be no need to see God in a purely theistic sense. If we take aesthetic pantheism out of the equation for a second then why can’t God be in everything, with God reflecting all matter and all matter reflecting God? We are made in Gods image according to Christians, if God is the creator then a bit of Him would lie in all his creations and all his creations would be part of Him. It’s probably better explained by the concept in the Kabbalah of sparks of light. In every particle of matter lies a spark of the divine.
Once we separate the spirituality of God and Christ from the myth stories surrounding them it is possible for a pantheistic Christianity to exist.
Why have so many died? Misappropriation And misunderstanding of Christ and God. How do we stop it happening again? Education, encouraging critical thought, stop people following gods (especially the secular gods like money, celebrity etc). Stop encouraging divisions, yes diversity policies, cultural appropriation I’m looking at you. I believe the answer lies in true esotericism (not internalised exotercism which is often sold as esotericism under the guiding light of some dubious leader). If you create a secular society there’s only ever going to be new gods created, blindly followed by people who are not encouraged to think for themselves.