Before you start the game, or start a new file, you will be asked questions at the start of the game. One of the questions is which god your character believes in. To have the god of Jupiter as a choice, you must beat the game at least once.
Fight your way through the game until you make it to Day 48. On Day 48, Marcia will tell you about Commodus and how he wants to train with you. Answer her with refusal. When she asks the second time, accept it.
When you are traveling to see Commodus, Marcia will ask you if it is time to liberate Rome. Answer her with a "No".
You finally get to Commodus, kill him. Later on, on Day 49, Laetus will come and ask the player to be the executioner. Answer him with refusal until the question is no longer asked.
Then comes Day 50. Do not leave your room. Instead, sleep for the whole day. You will now be past Day 50.
While at the Atilius Arena, there is that section that has a bunch of weapons like gladius's, wooden shields, etc. right before entering the arena. Pick up all of the weapons and store them in your weaponry box. Then walk over to the Arma (or market) and sell all of the items that you had just picked up. You won't get a lot of money by doing this, but it starts to add up after doing it a lot.
When you have to answer the questions before coming a gladiator, you have to choose where you come from.
You can unlock the locations of Rome and Greece by completing the game at least once. So if you ever start a second file, you will have more locations to choose from.
Once you start the game, or start a new file, you will be asked questions before you start. One of the questions will be what your character was before they were recruited to where they are now. To unlock Nobleman, Soldier, and Gladiator, you must complete the game at least once.