The majority of the conspirators do not support Brutus's decision to allow Mark Antony to speak at Julius Caesar's public funeral but eventually allow Brutus to have his way after he makes a convincing argument. Mark Antony is an avid supporter of Julius Caesar, and the conspirators discuss murdering him along with Caesar before their assassination attempt. However, Brutus insists that Mark Antony is harmless and argues that murdering him too will make them seem like butchers. Brutus convinces them to allow Mark Antony, to live and the conspirators do not attack him on the Ides of March.
Following Caesar's assassination, Antony's servant addresses the conspirators on his behalf, and they assure the servant that they will not harm Antony.
When Antony initially addresses the group of senators, he cleverly pledges his allegiance to Brutus and behaves amicably towards them. Antony then asks if Brutus will allow him to speak at Caesar's funeral in the marketplace and Brutus gives him permission. However, Cassius intervenes and tells Brutus,
You know not what you do. Do not consent/That Antony speak in his funeral. Know you how much the people may be moved/By that which he will utter? (Shakespeare, 3.1.244-249)
Despite the warning, Brutus assures Cassius that everything will be fine after he addresses the masses first. Brutus tells Cassius that Antony will only speak highly of them and his speech can only help their cause. Given Brutus's prominent role among the conspirators and his convincing argument, Cassius and the other senators trust his decision to allow Antony to speak at Caesar's funeral, which turns out to be a significant mistake.