In all the Gospels, the story of the calling of Levi and the meal with tax collectors and sinners follows the story about the forgiveness of sins that was connected to the healing of a paralytic. Apparently both events took place around the same time. In addition, there is a substantive connection. After Jesus spoke publicly about his right to forgive sins, he shortly thereafter speaks as publicly about his coming to call sinners to that forgiveness too.
Modern commentaries often explain Mark 2:13–17 in two distinct sections: a story of a paradigmatic calling (Mark 2:13–14), and an argument with a pronouncement by Jesus (Mark 2:15–17). The second section is then further subdivided into several subsections.1 However, Mark presents Mark 2:13–17 as a coherent unit. You can understand Mark 2:15 as a new beginning only if you proceed from a reconstructed text.2
The story itself already indicates the coherence of Mark 2:13–17. Jesus calls Levi and afterward there is a meal in Levi’s house with tax collectors and sinners. Although the number of the people thus increases, the story remains an event surrounding Levi’s call.
The meal in Levi’s house connects Mark 2:15–17 with Levi’s call (Mark 2:13–14). Therefore we have to read Mark 2:13–17 as a unity.3
13 He went out again beside the sea, and all the crowd was coming to him, and he was teaching them.