While it is true that the word 'apostate' refers to someone who leaves their religion, it is a very negative term. Why not say 'people who leave the organization' or 'former members'? Why use the word 'apostate'?
'Name-calling slaps a negative, easy-to-remember label onto a person, a group, or an idea. The name-caller hopes that the label will stick. If people reject the person or the idea on the basis of the negative label instead of weighing the evidence for themselves, the name-caller's strategy has worked.' -- See Awake!, June 22, 2000 pages 6 - 10.