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Jesus' closest disciples witnessed his miracles, teachings, crucifixion, and resurrection. They were with him throughout his ministry, witnessing his profound wisdom, healing powers, and the fulfillment of prophecies. Their experiences with Jesus shaped their faith and empowered them to spread his teachings after his death.
Those closest to Jesus was the apostles, and He was particularly fond of Peter.
Jesus
his 12 disiples and a couple of other people
he spread the word of Jesus throughout his life.
Peter was one of Jesus' closest friends and felt most comfortable with him.
Primarily, it was the Apostle Paul, selected to go to the gentiles.
There was no single person who spread Christianity throughout the empire. Each one of the apostles of Jesus went to different areas to preach. Paul also did a lot of writing and traveling and converting.
Everyone. Every person ever created, Jesus was sent to save them. Everyone.
This was Rome whose Government was mercilessly killing the followers of Jesus at one time and about a century after it. If anyone who was not a christian in Rome was considered to be an "evil". It bettered the condition of people in Rome. Gladiators were banned after it. Fighting with animals was also banned and many other reforms took place.
All Jesus' disciples spread Jesus' traditions. And they still do so.
Off hand, I would say that of the twelve disciples, John and Peter were His two closest friends. John, in his gospel called himself "the disciple whom Jesus loved". Peter was the one who after he had denied Jesus three times, was asked by Jesus: "Peter do you love me?" three times.