A blasphemer is a person who blasphemes: someone who speaks in a way that insults God or people’s religious beliefs, or who uses the names of God and holy things when swearing. It is a label, and a heavy one. In older religious writing it is almost a charge — “He is a blasphemer” — not a casual joke about a rude friend. You would not call someone this unless you meant they had crossed a sacred line. Though actually, people who do not share that faith may refuse the label altogether; it always belongs to a community that feels the insult.