A state that historians call declining can still look strong and well run to someone living inside it. An eyewitness may find officials competent and firmly in control of the provinces at the very moment the dynasty is supposedly falling apart. Close, everyday observation can correct the broad labels of later history.
A pilgrim sent abroad to fetch sacred texts brings back more than books. He brings training, methods of devotion, and even the correct sounds of a foreign alphabet, learned in person from a native teacher. These carried-home practices can then expand the influence of the movement he belongs to.
A religion can reach its highest point and then be violently suppressed within the same few years, when a hostile ruler takes power. An outsider who lives through both sees the faith as popular, everyday practice and then sees how quickly it can be torn down. Such a firsthand account becomes the best record of the turning point.