Because we do not know Mark’s sources (in contrast to Matthew and Luke, whose sources we either possess, i.e., Mark, or at least can extract, as in reference to Q), we cannot always be sure where the evangelist has added his unique comments to the story of Jesus. However, there are a few obvious places to look. The very first verse, which in antiquity is called the incipit, is a good place to start. In late antiquity the incipit more or less functioned as the title of a book.
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