Inward/Outward Change

Variations of the following phenomenon are all too frequent: A couple leaves (to some extent) family, friends, and their old life to join a conservative (in practice and belief) Christian church. They seem sincere enough, maybe even becoming more conservative than the average person at the church they have joined. After a number of years they become lax on some of their standards, grow disillusioned with the church, and leave it. They lose their distinctiveness from the world, do not attend another church, and eventually cease to profess Christianity at all. The last is worse than the first. Whose fault is it (theirs, the church's, both, neither)? Were they really sincere Christians to start with, did they honestly have a personal relationship with God? Or were they just into the "careful" and quaint lifestyle, the safer set of influences for their children? Was Christianity only a set of rules that made sense to them and provided a structure for their outward lives apart from a heart change?