MARCUS AURELIUS: There once was a dream that was Rome.
These words from Marcus Aurelius will be repeated throughout the film, usually by Maximus. The "dream" that Aurelius describes is the vision of a powerful Republic. This contrasts with the imperial desires of Commodus, who wants nothing to do with republican government.
COMMODUS: But these senators scheme, squabble and flatter and deceive. Maximus we must save Rome from the politicians, my friend.
Early on, we get the impression that the Roman Senate is the real villain, full of guys who "scheme" and "deceive." The irony, of course, is that it is the Emperor Commodus who begins to "scheme" and "deceive" shortly after these remarks. It is Commodus from whom Rome will need to be saved.
MARCUS AURELIUS: I want you to become the protector of Rome after I die. I will empower you to one end alone, to give power back to the people of Rome and end the corruption that has crippled it.
Marcus Aurelius glosses the idea of the "dream" here but also entrusts Maximus with a mission that he will, eventually, accomplish. The "corruption" he is describing has to do with issues of imperial government—the kinds of things Commodus gets involved with (politically motivated killings, for example). Maximus mission is to rid Rome of the disease, in addition to avenging his family's murder.
MARCUS AURELIUS: Commodus is not a moral man…Commodus cannot rule, he must not rule.
Marcus Aurelius wants politics to be pure and noble, not the haunt of morally questionable people. This is why he doesn't want Commodus to rule—he knows that evil people with power are the worst.
COMMODUS: That's the very problem, isn't it? My father spent all his time at study, reading books, learning his philosophy. He spent all his twilight hours reading scrolls from the Senate. All the while, the people were forgotten.
These are some of the most ironic lines in the film. Commodus pretends to be a politician that speaks for the people, and thus attempts to assume the position of the Senate (they're supposed to represent the people). This goes hand in hand with his attempts to eliminate the Senate, and is doubly ironic because Commodus only cares about the people insofar as they gratify his lust for attention and power.
COMMODUS: A vision. I will give the people a vision and they will love me for it. They will soon forget the tedious sermonizing of a few dry old men. I will give them the greatest vision of their lives
Despite the fact that he's totally nuts, Commodus is a shrewd politician. He and Gracchus understand how important the "people" are to the smooth running of the political machine, empire or not. Commodus knows that if he distracts the people with a "vision," they'll forget the "dry sermonizing" of the senators about republican government.
SENATOR GRACCHUS: Rome is the mob. Conjure magic for them and they'll be distracted. Take away their freedom, and still they'll roar. The beating heart of Rome is not the marble of the Senate, it's the sand of the Colosseum. He'll bring them death, and they will love him for it
Despite the fact that Rome's an empire, Senator Gracchus understands that the real source of political power is the "mob." Right now, Commodus has control of it, rendering the Senate's more benevolent intentions worthless.
COMMODUS: I think the time is almost right. I could announce the dissolution of the Senate at the celebration to honor our father.
Commodus wants total power, and he toys with the idea of ditching the Senate for good throughout the film. The irony, of course, is that Marcus Aurelius wanted to return power to the senate, and yet Commodus wants to dissolve it in honor of him.
SENATOR GRACCHUS: I will not trade one dictatorship for another
Gracchus is right to worry that Maximus will abuse the power he will gain by entering Rome with an army. However, he's so committed to the ideal of republican government, and he knows there are no other options. He has to make a leap of faith.
COMMODUS: For the health of Rome, the Senate must be bled. And he will bleed too.
Commodus' political ideology sees the Senate as a body that needs to be "bled." It's way of saying that the Senate is an infection, and for the "health of Rome" it must be killed, eliminated, and destroyed.
MAXIMUS: There was a dream that was Rome. It shall be realized. These are the wishes of Marcus Aurelius.
Good triumphs over evil, freedom over slavery, and republican government over tyranny. Marcus Aurelius' "dream that was Rome" is a republican government, with power vested in the Senate. Maximus' death realizes that dream, and the film leaves us with a vision of the sacrifices necessary for the establishment of a good, political order.