Kieregaard, Fear and Trembling

Kieregaard calls for absolute devotion and obedience and opposes faith to reason. But without sobering checks by reason such a faithful person is vulnerable to being made use of by various religious fanatics who do nothing but wait for such people to use them for their own despicable or insane purposes.
How serious is this problem for Kieregaard?
How does his belief that faith is not something social or interpersonal
(that it is a personal relation between the believer and God) bare on the objection?