This study is testing a medicine called **ritlecitinib** on people with **Celiac Disease (CeD)**, a condition where the body reacts badly to gluten, a protein in wheat. Participants must be 18 to 75 years old, have inactive CeD, and have been on a gluten-free diet for at least 6 months. They will take either ritlecitinib or a placebo (a fake pill) once a day. Everyone will also eat 10g of gluten each day for 21 days, mixed into food like hot chocolate or apple sauce. If 10g is too much, participants can switch to 5g after the third day. - **Duration:** Study lasts for 21 days with daily medication and gluten consumption. - **Visits/Tests:** Participants need a negative COVID test before starting and two more before endoscopy (a test to look inside the stomach). - **Important Considerations:** Participants must avoid strenuous exercise, excessive grapefruit juice, and use effective birth control if needed.