If Christians tend to be gullible especially in regards to their faith, they typically hold themselves to a high standard of honesty and scrupulousness with regard to worldly affairs. Thou shalt not steal. Thou shalt not bear false witness against thy neighbour. This is not to say that every Christian is a paragon of secular virtue, only that the standard itself is particularly demanding in non-religious matters. At a minimum this is how the vast majority of Christians like to see themselves.
From time to time, someone, often a young person, comes along and innocently applies the honesty and scrupulousness which they had been taught to their faith. And what is the result? Atheism. Is atheism the logical consequence of Protestantism and perhaps Catholicism as well? Christian ethics overcoming Christian dogma? It is, after all, a common trope for atheists to read and question the Bible more closely than do Christians.