Looking more into this, I fail to find evidence that such a tithe played a large role or that it is something that came about peacefully or naturally. From Wikipedia:

When Olaf Tryggvason ascended to the crown of Norway, the effort to Christianize Iceland intensified. King Olaf sent an Icelander named Stefnir Thorgilsson back to his homeland to convert his fellow countrymen. Stefnir violently destroyed sanctuaries and images of the heathen gods — this made him so unpopular that he was eventually declared an outlaw. After Stefnir's failure, Olaf sent a priest named Thangbrand (Old Norse: Þangbrandr). Thangbrand was an experienced missionary, having proselytized both in Norway and the Faroe Islands. His mission in Iceland from c. 997-999 was only partly successful. He managed to convert several prominent Icelandic chieftains, but killed two or three men in the process.[citation needed] Thangbrand returned to Norway in 999 and reported his failure to King Olaf, who immediately adopted a more aggressive stance towards the Icelanders. He refused Icelandic seafarers access to Norwegian ports and took as hostages several Icelanders then dwelling in Norway. This cut off all trade between Iceland and its main trading partner. Some of the hostages taken by King Olaf were the sons of prominent Icelandic chieftains, whom he threated to kill unless the Icelanders accepted Christianity.

The Icelandic Commonwealth's limited foreign policy consisted almost entirely of maintaining good relations with Norway. The Christians in Iceland used the King's pressure to step up efforts at conversion. The two rival religions soon divided the country and threatened civil war.
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it very much supports the idea that I already presented that it was the increasing power provided to those "private defense companies" or the kings which made such changes possible.

300 years is not "quickly".

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Neither did Iceland start from a situation PP and inequality. Remember, it started with Communal living and possession. The "quickly" was going for the case of moving our current society to such a private defense system.

As far as we can tell the Icelanders were not coerced via force into imposing the tithe. Perhaps they were duped, but this is not the fault of their original political system.

In the case of Hitler I couldn't say it better than this

On January 30, 1933, President Hindenburg appointed Adolf Hitler chancellor of Germany. Although the National Socialists never captured more than 37 percent of the national vote, and even though they still held a minority of cabinet posts and fewer than 50 percent of the seats in the Reichstag, Hitler and the Nazis set out to to consolidate their power. With Hitler as chancellor, that proved to be a fairly easy task.