The way I see it, there's two ways to go about this.
You can either force the Legionaries to see not just your frail human form when they look at you, but a hundred well-armed cultists who are more loyal to you than to them. In the BC ruleset, the gap between Astartes and Human weapons is notably closer than in the other games of FFG's 40k line, which means that even though one CSM versus one Human has a huge advantage, he's quite killable when facing an entire squad of your elite bloodsworn bodyguards. Depending on the characters' personalities and everyone's usefulness to their fellow heretics, this might even have your Apostate rise to a leadership position, as long as you manage the tricky task of keeping your Traitor Marines happy and thus loyal.
Of course, the downside is that this approach also comes with a significant risk of forcing internal conflict and scheming if your character fails to convince the Legionaries of them being better off with him at the helm, or at the very least a sort of council of equals (where you use your cunning and silver tongue to get the better of everyone else, in spite of this supposed equality).
Alternatively, you could also just suck it up, realise that - with the exception of psykers - this system still has a habit of sabotaging the idea of capable Humans measuring up to Astartes (though not nearly as much as the other games, as mentioned above), and try to settle into a supporting position where you may not call the shots on everything, but still live comfortably in a position of power at the mercy of your CSM overlords, who have recognised your usefulness and are willing to suffer your antics in exchange for the influence you are able to project onto the unwashed masses.
Both paths could make for a fun game - it really depends on what you wanted to get out of this character when you created him.