The first book in the New Testament is the Gospel of Matthew. It's author was a tax collector until he was called by Jesus to be one of His 12 disciples.
Matthew's Gospel was written about 20-30 years after Jesus had ascended to heaven. During that time the good news had spread throughout the Roman Empire, but Christians were starting to be persecuted. The aim of Matthew's Gospel was to strengthen the faith of Jesus' followers during these hard times, and also to give them a tool to help them spread their faith to others.
Matthew 9:27-34
"And when Jesus departed thence, two blind men followed him, crying, and saying, Thou Son of David, have mercy on us. And when he was come into the house, the blind men came to him: and Jesus saith unto them, Believe ye that I am able to do this? They said unto him, Yea, Lord. Then touched he their eyes, saying, According to your faith be it unto you. And their eyes were opened; and Jesus straitly charged them, saying, See that no man know it. But they, when they were departed, spread abroad his fame in all that country.
As they went out, behold, they brought to him a dumb man possessed with a devil. And when the devil was cast out, the dumb spake: and the multitudes marvelled, saying, It was never so seen in Israel. But the Pharisees said, He casteth out devils through the prince of the devils"