Is there any known reason that Christianity of all religions spread so successfully throughout the Roman empire? From what I understand, it started out as a Jewish sect, so why did THAT one take off so well while other ones didn't?
From a brief wikipedia browse it seems to be because of this dude.
Looks like his sons were Christian too and the guy who tried to bring Zeus' sexual habits back got completely fucked in Persia. Also, the emperors following Julian did stuff like this.
Fucking Ninjas.
Constantine is not the explanation on its own. Possibly without him it wouldn't become as successful as it was, but it had to get to a point he could have converted in the first place.
Inclusiveness is a big part, definitely. They wouldn't have converts if they couldn't have them, so the policy to accept gentile converts helped.
Early Christianity also had the advantage of having guys like Paul of Tarsus, who was familiar with the Roman culture and thought and so knew how to approach Romans in the first place.
There's also the ideological component. Apparently, Christianity was pretty big for Roman women in particular, since it was relatively egalitarian compared to the Roman society. Roman marriage was basically CK2-style political marriage
up to eleven.
The fact that RE gave few fucks about who the citizens worshipped in general also might've helped, until the whole business with not worshipping the Emperor and the persecutions and all (fun fact: Blood Libel was originally used against
Christians... who promptly co-opted it against Jews as soon as they got into power and had a fever with the only prescription being pogroms).