I was searching around on the Internet the other day and found this picture of a Greek text from Matthew and Luke. This passage is Matthew 3:7-10 and Luke 3:7-9. All of the words in red are where these two gospels show perfect agreement.
This much agreement in the text is strong evidence for the synoptic gospels sharing a common source. Agreement this close couldn’t have come by just chance. The authors had to copy from the same source. Many other passages in the synoptic gospels also share this kind of similarity.
Passages that share this much agreement in their wording are also evidence for the authors using more than just an oral source. It is evidence for a written source. There is the possibility of an oral tradition changing over the years. However, once a tradition is written down, it is unchangeable. Therefore, if there is a close agreement in the wording (such as this text), then there is greater the evidence for a literary source.
I feel that this along with the other arguments I have shared are a strong evidence for a literary relationship between the gospels.
