An interesting debate, but one IMHO will never lead to a conclusion.
I was brought up in a fundie evangelical sect (PB) which thought that the vast majority of believers outside the sect were not 'real' xians. That included RCs, CofE, etc. etc. And those were only the ones they thought they knew anything about. Orthodox, Ebionites, Marcionites, and other (to them) unknowns must have been heretics.
I am a keen but amateur student of the history of the early Jesus followers. Even my shallow reading shows just how diverse the early beliefs were. (This is why the claims of WT and other sects to be following the 'true earliest beliefs' are untenable after even basic research).
I like Bart Ehrman's work and find it thought-provoking. This is his view on 'what is a Christian':
" I think anyone who wants to call themselves Christian ought at the very least to (1) believe there is some kind of God in the world (a superior divine being of some kind) and (2) think that Jesus somehow reveals that God or makes access to that God possible or … or anything else that makes Jesus important in relationship to that God."