Question

Describe a way in which Christianity spread after Jesus' death.

Answer 1

Forty days after the death of Jesus, God gave the apostles the gifts of the Holy Spirit. This event is currently known as the Pentecost. This event led to the apostle taking courage to spread Christianity to neighboring countries. 

Answer 2

Peter and Paul traveled throughout the empire spreading the message of Jesus.
Constantine and the Council of Nicaea had nothing to do with early Christianity. Christianity had already been around nearly 300 years by that time.
And it was Constantine, not Nero, who issued the Edict of Milan.