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 7:13-27
"Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat: because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.
Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves. Ye shall know them by their fruits. Do men gather grapes of thorns, or figs of thistles? Even so every good tree bringeth forth good fruit; but a corrupt tree bringeth forth evil fruit. A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit. Every tree that bringeth not forth good fruit is hewn down, and cast into the fire. Wherefore by their fruits ye shall know them.
Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock: and the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not: for it was founded upon a rock. And every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand: and the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell: and great was the fall of it"