In this excerpt from the Eric Hoffer's book, The True Believer, we are given insights into some of the sources of missionary zeal.
Whence comes the impulse to proselytize?
Intensity of conviction is not the main factor which impels a movement to spread its faith to the four corners of the earth: 'religions of great intensity often confine themselves to condemning, destroying, or at best pitying what is not themselves." Nor is the impulse to proselytize an expression of an overabundance of power which as Bacon has it "is like a great flood, that will be sure to overflow." The missionary zeal seems rather an expression of some deep misgivings, some pressing feeling of insufficiency at the center.
Proselytizing is more a passionate search for something not yet found than a desire to bestow upon the world something we already have. It is a search for a final and irrefutable demonstration that our absolute truth is indeed the one and only truth. The proselytizing fanatic strengthens his own faith by converting others. The creed whose legitimacy is most easily challenged is likely to develop the strongest proselytizing Impulse. It is doubtful whether a movement which does not profess some preposterous and patently irrational dogma can be possessed of that zealous drive which "must either win men or destroy the world." It is also plausible that those movements with the greatest inner contradictions between profession and practice -that is to say with a strong feeling of guilt - are likely to be the most fervent in imposing their;faith on others. The more unworkable communism proves in Russia, and the more its leaders are compelled to compromise and adulterate the original creed, the more brazen and arrogant will be their attack on a non believing world. The slaveholders of the South became the more aggressive in spreading their way of life the more it became patent that their position was untenable in a modern world. If free enterprise becomes a proselytizing holy cause, it will be a sign that its workability and advantages have ceased to be self-evident.
The passion for proselytizing and the passion for world dominion are both perhaps symptoms of some serious deficiency at the center. It is probably as true of a band of apostles or conquistadors as it is of a band of fugitives setting out for a distant land that they escape from an untenable situation at home. And how often indeed do three meet, mingle and exchange their parts.