Miracles began to happen after Jesus instructed his disciples to preach the gospel everywhere and he told them that miracles would accompany their ministry.

Mark 16:15-20

15 Later he said to them, “Go into the whole world and preach the good message to everyone! 16 Everyone who believes your message and who is baptized will be saved {God will save}. But everyone who does not believe it will be condemned {God will condemn}. 17 Those who believe my good message will perform miracles. Specifically, by my power they will expel evil spirits. They will speak in languages that they have not learned. 18 If they pick up snakes accidentally or if they drink any poisonous liquid accidentally, they will not be hurt. Whenever they put their hands on sick people in order that God will heal them, those sick people will become well.”

19 After the Lord Jesus had said this to the disciples, he was taken {God took him} up into heaven. Then he sat down on his throne beside God to rule with him. 20 As for the disciples, they went out from Jerusalem, and then they preached everywhere. Wherever they went, the Lord enabled them to perform miracles. By doing that, he showed people that God’s message is true.