The Question and Answer section for Letter From Birmingham Jail is a great resource to ask questions, find answers, and discuss the novel.
King accuses the clergymen of not taking the cause of equality seriously. He says that their words ring hollow if there is no action behind them. King asserts that there must be active protest if they are ever to get their message out.
Can you please provide a short quote from the lines you mention so I know exactly where you are referring to?
“Letter from Birmingham Jail” is addressed to several white clergymen who had written an open letter criticizing Dr. King's actions as well as the Southern Christian Leadership Conference (SCLC) during their protests in Birmingham.