Among the crowd listening to his teaching was Levi, a tax collector (therefore a collaboration with the Romans, the occupiers of Israel), Jesus approached him and said, "Follow me." Levi did.
Jesus had (hosted?) a meal that included many tax collectors and sinners. The religious insiders expressed disapproval. Jesus responded, "Those who are well don't need a doctor. I haven't come to summon the righteous. I'm here for the sick."
Note: Morna Hooker in her commentary, The Gospel According to Saint Mark, says "Levi, son of Alphaeus, is not included in the list of the Twelve in 3:16-19, although there is a reference to a 'James, son of Alphaeus'. This discrepancy has been solved in some manuscripts by the substitution of James' name for Levi's."