Why did the Roman Empire suddenly believe in Jesus after previously killing him?

 The question seems to have a misunderstanding. The Roman Empire, as a political entity, did not collectively believe in Jesus. Instead, it was individuals within the Roman Empire, particularly after the events of Jesus' crucifixion and the subsequent spread of Christianity, who came to believe in Jesus as the Son of God.

The crucifixion of Jesus took place during the Roman occupation of Judea, and it was carried out by Roman authorities at the request of some Jewish leaders. The Roman Empire, as a ruling power, was not directly involved in religious matters or belief systems. However, after Jesus' death and the spread of his teachings by his followers, Christianity began to gain followers within the Roman Empire.

Several factors contributed to the spread of Christianity within the Roman Empire:

1. **Missionary Efforts:** The disciples of Jesus and early Christian missionaries played a crucial role in spreading the message of Christianity to various parts of the Roman Empire.

2. **Emperor Constantine's Conversion:** In the early 4th century, Emperor Constantine converted to Christianity and issued the Edict of Milan in 313 AD, which granted religious tolerance to all religions, including Christianity. This marked a significant turning point for the acceptance of Christianity within the Roman Empire.

3. **Council of Nicaea:** In 325 AD, the First Council of Nicaea was convened, bringing together Christian bishops to address theological issues and establish doctrinal consensus. This helped solidify certain aspects of Christian belief.

4. **Official Recognition:** In 380 AD, the Edict of Thessalonica declared Christianity, specifically Nicene Christianity, as the official state religion of the Roman Empire under Emperor Theodosius I.

It's important to note that the process of Christianization within the Roman Empire took several centuries and involved a combination of social, political, and cultural factors. The initial followers of Jesus were often persecuted, but over time, Christianity gained acceptance and eventually became the dominant religion in the Roman Empire.