In this scene Artemidorous is trying to warn Caeser not to trust Brutus, Cassius, Casca, Cinna, Trebonius.(All the conspirators)So he writes him a letter to warn him naming all the conspirators. The thing about this is that to accept the warning, Caesar must break the trust found in each of the conspirators, and he must fall humble and lose his overconfidence in himself. This does not happen, and thus Caesar is assassinated.
Artemidorus` letter from the original text.
“Caesar, beware of Brutus. Take heed of Cassius.
Come not near Casca. Have an eye to Cinna. Trust not
Trebonius. Mark well Metellus Cimber. Decius
Brutus loves thee not. Thou hast wronged Caius
Ligarius. There is but one mind in all these men, and
it is bent against Caesar. If thou beest not immortal,
look about you. Security gives way to conspiracy. The
mighty gods defend thee!
Thy lover,


“Caesar, beware of Brutus. Watch Cassius. Don't go near Casca. Keep an eye on Cinna. Don't trust Trebonius. Pay attention to Metellus Cimber. Decius Brutus doesn't love you. You've wronged Caius Ligarius. These men all have one intention, and it's directed against Caesar. If you aren't immortal, watch those around you. A sense of security opens the door to conspiracy. I pray that the mighty gods defend you!
Your friend,
Barns and nobel. "No Fear Shakspear" 28 april 2009.