The meaning of the poet's saying: "So the best of your army, from them you are disavowed, when they drew near to faith. These armies have met openly, and the battle has drawn near, and the call has been made"

The Shaykh : He said: "So the best of your army, from them you are disavowed, when they drew near to faith"
This means that among the Mu'tazilah, some of their imams have drawn near to the truth, but despite that, they do not accept it. They mention their statements as being abandoned sayings.
"Line up the armies and deploy them, and go forth for war and draw near to the horsemen"
They are, yes.
"These armies have met openly, and the battle has drawn near, and the call has been made to the equals"
This means that now the issue is depicted as if the meeting has occurred, the battle has drawn near, and the call has been made to the equals. The equals are those who are matched in courage and boldness.
"Line up the armies and deploy them, and go forth for war and draw near to the horsemen
They long to meet you with longing, so that they may fulfill their vow of sacrifice.
(May Allah have mercy on him). Why? For the sake of killing. They are longing to meet you in order to draw near to Allah by killing you.