What is the most important theme in the play Julius Caesar?
It might be argued that the dominant theme in Julius Caesar is fatalism. This word is defined as:
The belief that all events are predetermined and therefore inevitable.
Shakespeare was working with the actual events of history. Everything that happens in the play is already predetermined. Not only will Caesar meet his death on the Ides of March, but Antony will make a stirring funeral speech, Brutus and Cassius will commit suicide on the battlefield at Philippi, and everything else will work itself out exactly as fate ordained at the beginning of time.
Shakespeare shows men trying to shape the future, thinking that they have the power to do so. But from our perspective what happened in Rome before, during and after Caesar's assassination was inevitable. The events have been sealed by time. That is one of the interesting features of Julius Caesar. What we see happening has already happened. The actors playing Caesar, Brutus, Cassius, Antony, and others are only actors. They have no power to change the outcome of the play.
In Act 5, Scene 1 Brutus seems fatalistic when he says:
O, that a man might know
The end of this day's business ere it come!
But it sufficeth that the day will end,
And then the end is known.
There are some contemporary thinkers who suggest that, since every effect must have a preceding cause, everything that has happened and will ever happen was predetermined at the time of the Big Bang. Like the doctrine of Solipsism, it is an interesting concept for the mind to play with but not subject to proof.
While there are a number of prominent themes in Julius Caesar, including ambition, manipulation, and competition; but it could be said that the most significant theme is humility. Accepting one's flaws is key to self-actualization (a state of mind not often achieved by Shakespeare's protagonists).
In Act I, scene 2, Cassius says to Brutus: "The fault, dear Brutus, is not in our stars, But in ourselves, that we are underlings." Brutus is troubled by Caesar's rapid rise to prominence and wants to prevent him from becoming emperor. Cassius reminds him that Caesar is only a man, not a god, and that their own social ranking beneath him is their own doing and not Caesar's fault. Cassius is suggesting that their sense of self-worth is their own responsibility, and cannot be conferred by someone who may be perceived as being superior to them in a social context. But just as this conversation could be said to help convince Brutus his desire to destroy Caesar is righteous, it is also a potent reminder that he can choose his own destiny, and just as easily choose a path that is more morally sound.