The basic problem that King expresses with the white church in this letter is that they think that order is more important than justice. They would prefer, he says, that King and other blacks wait and let justice come to them in due time, without causing disorder. He tells them that this is disappointing to him because he can understand those who actively oppose him but has a harder time understanding those who claim to support justice but won't support the methods needed to achieve it.
King is disappointed with the white moderate churches because they want him to wait patiently and keep order even if that means suffering from injustice.
In his "Letter from Birmingham City Jail," Martin Luther King Jr. expresses considerable disappointment with the white church for a number of reasons. First of all, he is disappointed that the white ministers and priests have not offered their support to his campaign for racial equality. King Jr. believed that they would be among his "strongest allies" but, in fact, they have formed some of his most staunch "opponents" because they failed to understand the aims of the movement and often "misrepresented" the views and actions of its leaders.
Secondly, he is also disappointed that many white churchmen have failed to see the importance of the Gospel (and of religion, more generally) in this movement. To support this idea, he quotes some ministers who have stated that this movement is a "social issue" with which the Gospel has "no real concern." For King Jr., this is disappointing because he is trying to encourage unity and harmony between races which, in his opinion, is a cornerstone of the Christian religion.