Spoiler:
i dont know....lindy told cloc about how sentry was just an addict and will always remain one(he is addicted to the power now). But then void laughs at her an says that he would love to hear more of her theories. So it doesnt really seem like that is the truth about sentry/void. Void was obviously there in biblical times, dont know about sentry tho. Maybe when Robert Reynolds drank the serum he got the powers of the void, but since he was an addict and crazy he created the sentry so he could be a hero like captain america(like always wanted to be)He also refers to sentry as god himself a few times. It really seem to me that he is some sort of wrath of god at this point. Maybe he will seem like the villain in siege so that asgard and earth unit to fight a common foe. Which will in turn end the siege of asgard.
Or he will do some crazy black plague thing again like he did in biblical times to end all the fighting...just some thoughts
Im also seeing sentry/void as something similar to the phoenix force, and bob reynolds might just be one of many hosts.