1) Christianity founded by Jesus is a heretical faction of Judaism. Strictly speaking, there was no Christianity before Jesus, including the Bible, the Old Testament and the Prophet Moses, which belonged to Judaism in the historical sense. Jesus was murdered by Jewish conservatives because he reformed Judaism.

2) Christianity founded by Jesus, or Judaism reformed by Jesus, is the first religion in western history that advocates "equality of all beings regardless of nationality and class". Before Jesus, all western religions either emphasized national origin (for example, Judaism emphasized that only Jews could be saved); Or emphasize class composition (such as ancient Greece and Rome, each social class has its own belief in gods). Jesus was the first to put forward the concept of "all original sins are equal", without distinction and equality. So Christianity is particularly popular among the lower classes.

3) At the same time, due to the rise of the Roman Empire, the population flow within the Empire has greatly increased, making the psychological impact of social inequality stronger than before.

For example, wealthy Egyptian businessmen under Roman rule are highly respected in Egypt; When he went to Rome for business, he became a humble foreigner. The huge gap brought by this status gave people a greater psychological impact in the Roman Empire than in any previous era.

Therefore, Christianity, which emphasizes the equality of all beings, is not only popular among the lower classes, but also spread to all parts of the empire as people of different classes move within the empire.

I don't understand the second question. ...

The third question:

1) Before Constantine the Great ascended the throne, the empire was ruled by four emperors, that is, Rome was divided into two parts, east and west, and each part was divided into two emperors. At that time, Christianity was widely spread among soldiers in Rome. Because the Roman Legion at that time was already a hodgepodge of people of different nationalities and classes, the Christianity of "equality of all beings" was very conducive to resolving contradictions and enhancing cohesion within the Legion.

Therefore, winning the support of Christian missions is very important for gaining the loyalty of the army. In the process of seizing the throne, Constantine deliberately catered to various Christian customs and superstitions (including claiming that he dreamed of God, etc. ) and strengthened the loyalty of soldiers to him through superstitious activities.

2) After Constantine ascended the throne, in order to get rid of the interference of the old imperial power and strengthen the absolute monarchy, two measures were taken. One is to move the capital from Rome to the newly-built Constantinople; The second is to take Christianity as the new state religion, replacing the "Jupiter belief" of the old state religion in Rome.

3) Constantine the Great converted to Christianity in his later years. First, he followed the basic strategy of "consolidating rule with Christianity" for most of his life. Secondly, for the sake of the long-term stability of the empire, Christianity was established as the state religion through the "emperor's conversion", which gave the country a new spiritual pillar and enhanced the centripetal force of the country.