What are the causes of Jesus Christ about spread of Christianity?

Answer 1

On the day of Pentecost Peter preached the gospel message to people who were gathered in Jerusalem. (see Acts chapter 2 )

These people were Jews and Jewish converts from all over the known world. Many believed the gospel message and became Christians. When they went back to their home countries no doubt they told others, and so the message of salvation through Jesus Christ was spread.

ANOTHER ANSWER:

Paul was "commissioned" to do it, as Jesus informed Ananias:

"...the LORD said, 'Go and do what I say. For Saul is My chosen instrument to take My message to the Gentiles and to kings, as well as to the people of Israel. And I will show him how much he must suffer for Me.'" (Acts 9:15-16 NLT New Living Translation)

Plus, you may not have noticed that most of Jesus' disciples suddenly "disappeared" from the Scriptures after His ascension into heaven... because He had "commissioned" them, also, to spread the gospel [Good News]:

"Then the eleven disciples left for Galilee, going to the mountain where Jesus had told them to go. When they saw Him, they worshiped Him - but some of them still doubted! Jesus came and told His disciples, 'I have been given Complete Authority in heaven and on earth. Therefore, GO AND MAKE DISCIPLES OF ALL THE NATIONS, baptizing them in the name of the Father and the Son and the Holy Spirit. Teach these new disciples to Obey all the Commands I have given you. And be sure of this: I AM with you always, even to the end of the age.'" (Matt.28:16-20 NLT)

"...'WITH MY AUTHORITY, TAKE THIS MESSAGE of Repentance to all the nations, beginning in Jerusalem: There is forgiveness of sins for all who turn to Me.' You are witnesses to all these things. And now I will send the Holy Spirit, just as My Father promised. But stay here in the city until the Holy Spirit comes and fills you with Power from heaven.'" (Luke 24:47-49 NLT)

So, Jesus Christ "caused" the spreading of the gospel by "commanding" and "commissioning" it to be done by His own Divine Authority.

Answer 2

Jesus Christ preached about repentance and the kingdom of God, and afterwards his disciples preached the resurrection of the dead through our Lord Jesus Christ.

Jesus Christ caused this message to be spread by dying on the cross for our sins, and rising again from the dead and ascending up into heaven.

Answer 3

People were called to the truth of God and many people did not reject God. They became Christians instead of rejecting salvation from God.