Whisk together the molasses, vinegar, honey, brown sugar, red pepper flakes, ginger, garlic, salt, and pepper until combined. Remove 1 cup of this mixture to use for basting later. Pour the remaining marinade into a resealable plastic bag, and add the pork tenderloin. Coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 30 minutes to overnight.