Christianity has many similarities when compared to other ‘rival’ religions at the time such as Mithraism and obviously Judaism. Not only this but Christianity’s preaching and condemning of earthly pleasures and such may seem off putting to many casual observers compared to the numerous cults and what they could offer at the time. What gave it such an advantage earlier on to gain initial followers, so that even today it’s arguably the most widely practiced and influential religion in history?
More can be written, but you might like to start with "How did Christianity go from a small, hated cult to the state religion of the Roman Empire? What made it so different than the hundreds of other small cults throughout the Empire?" by /u/talondearg. This is out of the FAQ section "The Rise of Christianity".
This is not to discourage discussion. More questions, data and debate are welcome.