I haven't personally had a conversation with all the first century Christians, so it would be presumptuous for me to assume to know what they all believed.
I'm sure some of them misunderstood the Bible, just like some Christians do today.
That many Jews misunderstood the scriptures is clear from what Jesus taught. He quite openly contradicted the teachings of the Jewish religious leaders. Throughout the entire history of the Jewish nation there were false teachings and false practices corrupting pure worship. To adhere to a teaching just because "the Jews used to believe it" isn't a wise course of action.
Jehovah regulated slavery and Jehovah regulated polygamy, but that doesn't mean Jehovah approved of either of those things. Jehovah knew the people were pig-headed and hard-hearted, and He made provisions for the culture of the day. Just because the ancient Jews had slaves or multiple wives, would you do that today? Would Jesus approve? No, he wouldn't.
There is life under the ground today, and it's called bacteria, moles, worms, etc. The Greek mythological teaching of Sheol and Tartarus is different than the situation the Bible describes regarding Sheol and Tartarus. The faithful apostles said not to pay attention to stories like old women tell because those stories were silly and were already going around in the first century and prior to that. You can believe them if you want, but I'm not really into believing the mythology/fable thing. (I'm not into sci-fi either, like all that "evolution theory" make-believe where they use fancy words and high-sounding language to pretend stuff happened when there weren't any witnesses.🙂 Evolution myths are perpetuated in the same way as the 1914/FDS doctrines😜.)