Now that you have been inducted into the monastery, you will have to begin acting like a proper monk.
Take the tour with Antonius if you wish, but you can choose to skip it to speed things along. Life as a monk inside the monastery is not the most exciting and certainly does not compare to hunting bandits and Cumans. When you are done with Antonius, seek out the other novices and get to know them. You are looking for any indication that one of them might be Pious. At this stage you will most likely find them in the courtyard gardens.
Once Antonius has admitted to being Pious he will suggest a plan of escape. You can either choose to kill him there and then or agree to the plan in order to bring him back to Rattay. If you agree to the plan you will need to acquire the keys to the monastery from the Prior. These can either be pickpocketed from him, or you can find them on a shelf in one of the back rooms behind the kitchen.
When you have found the keys and ordered the blood, follow the other monks to the sleeping area and go to bed until dawn. When you wake up, the first thing you should do is collect the blood from the kitchen.
Once you have collected both keys and blood, you will have to eat with the others and continue the day as normal by completing your duties. After evening mass, meet with Antonius in the dormitory or follow him there by noting when he leaves mass early.