Final answer:
Jesus came into the world to bear witness to the truth.
Step-by-step explanation:
The reason that Jesus came into the world, according to John 18:28-40, is bear witness to the truth. In this passage, Jesus states, "For this purpose I was born and for this purpose I have come into the world, to bear witness to the truth." The context of the passage is Jesus being questioned by Pontius Pilate about his claim to be a king. Jesus clarifies that his kingdom is not of this world and that his mission is to testify to the truth.