A natural consequence of human existence is disagreement. Each of us tend to have opinions or beliefs as to the truth on any matter in question. Our interactions with others almost inevitably reach some contention as it is often the case that individuals have different beliefs and opinions on the same matter.
...the question is: How did they know who Jesus was? A couple possibilities came to mind.
Arguments are necessarily appeals to authority. When we make an argument in a debate, we inevitably appeal to some authority to validate our claim, whether it be a person, a publication, or propaganda!