I would say that most of the novel's atmosphere is foreboding and tense. Those won't work for the entire novel, because the concluding chapters bring a return to normalcy and calmness when Peter Solomon explains a bunch of details regarding the Lost Symbol to Robert Langdon.
The rest of the novel, in typical Dan Brown style, is paced quite quickly and an overall tense and foreboding atmosphere is created. Mal'akh helps establish the foreboding atmosphere because the guy is equally dangerous and scary. He has tattoos covering his entire body, and he shows no reservations about killing people. The sequence when he locks Langdon in the sensory deprivation tank is incredibly tense because readers know that Langdon suffers from extreme claustrophobia. Another extremely tense part of the book is when Mal'akh is hunting Katherine Solomon through the absolute darkness of her Noetic Science research facility. Darkness has a way of creating tense atmospheres to begin with. Now add being hunted to it. It would be terrifyingly tense.