Scholars who study the ancient world sometimes look at the New Testament to find historical facts. Historians who read the New Testament would consider it to be literature rather than history. The authors of this literature wrote in a different time period than we are living in, and had different standards for historical writings. They had a different goal in mind rather than recording exactly how many people attended a certain event or on what day it was that Jesus said a certain thing. The truth is, a few generations after the death of Jesus, there was an oral tradition in which the story of Jesus told. Certain writers, such as Mark, Matthew, and Luke all decided that they needed to record this information so that it could be more easily accessible and permanent. The point is, the goal of the authors was to spread the message and record a story. By the time all the accounts of the life of Jesus were written down, the historical facts were fuzzy or perhaps misinterpreted. This is where historians come in and try to filter out what can be taken as historical fact and what is inaccurate.
There are several methods that can be used to try to weigh the truth out of the New Testament. Historians will consider the source from which the writer got the information. Mark wrote down stories from oral tradition, but he also was influenced by what the early church was saying or believed about Jesus. This is where the criterion of dissimilarity comes in. This is the most reliable method for filtering out historical facts from the ancient writings. Using this criterion, the early writings about Jesus are compared with what they knew the early church believed or with other existing writing of Jesus. If the writings conflict with what the church was teaching at the time, there is a greater possibility that it is true. This is because the church would eventually adopt the stories about Jesus into the canon even though it might conflict with what they had formerly preached. Before the early Christian church was organized, there were probably many conflicting versions of stories going around, but eventually they came to a harmony presumably because they were historically accurate.
However, if it is known that the early church taught something, and it appears the same throughout the gospel, then its historical legitimacy is actually questioned. It is assumed that the writers were just acting as parrots and repeating what they knew was already believed. For example, Paul wrote that at the last supper, Jesus said "Do this in memory of me." (1 Corinthians 11:24) Luke, who wrote later than Paul, included this very same phrase (Luke 22:19). It happens that the early church was also teaching this very same phrase. Paul may have included this story from oral tradition and the church may have started teaching it too, so Luke liked the way it sounded and put it into his gospel. The writers would only write something conflicting, or dissimilar if they felt they were bringing some new more accurate information to light.
For example, as it explained in Acts 18:24-19:10, the movement of John the Baptist was in competition with the Jesus Movement. The disciples of John the Baptist were actively converted to the Jesus movement by Paul in this passage. If there were any tension between John the Baptist and Jesus, some stories of John the Baptist acting subordinate to Jesus in the gospels might try to set the early followers of Jesus straight, but it is hard to trust because the church and the gospel writers both had motivation to fabricate such stories. The church and authors of the New Testament believed that Jesus was the lord, and wanted the followers of John the Baptist to believe this too. However, the story of Jesus getting baptized by John the Baptist was likely true, because in a way it makes Jesus subordinate to John the Baptist and conflicts with the image of Jesus that the church and writers of the gospel were teaching. Therefore, Jesus's baptism passes the criterion of dissimilarity.