Combine the honey, spicy mustard, and apple cider vinegar in a small bowl.
Place the pork loin in a zip lock bag and add the marinade.
Zip the bag closed and marinate the pork loin overnight in the refrigerator.
When your grill is hot, remove from the refrigerator and cook over medium high. You can turn the pork loin once during cooking to get a nice crust on all sides. Close the lid when you’re not turning the meat.
Remove when meat reaches an internal temperature of 145 degrees. (No, it doesn’t actually have to get up to 160 degrees. That’s for other types of pork. The USDA says that 145 degrees is the perfect temperature for pork loin.) Cooking times will vary based on your grill so always go by a good internal temperature to gauge doneness.