Believing is more than just believing in one's mind or having a feeling, or perhaps a cheese driven lucid dream. The simple terms the word ‘belief’ in Greek is believing in the sense of trusting. The point here is that we trust Yeshua's (Jesus’) work on the cross alone for our salvation. Anyone who trusts anything else for salvation the Bible says is believing a false gospel. Commonly these can be 'systems of works' by human effort, of a focus on something idolised. There are many religious cults pertaining to be Christianity, they can be identified because they display strict rules and laws that may involve penance, works and blind or irrational obedience. This all happens in a way that is highly non-congenial to family and friends and the broader community. You can easily identified false Christian belief if the protagonists say that certain things must be done in order to win salvation or favour with God.