King called for a new "army" to face the "marvelous new militancy" that had "engulfed the Negro community." What was unusual about that army, given the nature of his sermon?

He's referring to the people accompanying the struggle

Explanation:

For King, it was a positive fact that many white people was starting to realize that their fight was not only just, but bound to their own freedom. So, the new militancy he mentions in the speech is the white people accompanying, in the streets and in the crowd receiving the speech.