What was the question of religious principle that gave rise to the war between Lilliput and Blefuscu?

In the novel Gulliver's Travels, Gulliver lands on the island of miniaturized people Lilliput, and finds them very trivial in nature. They are constantly at odds with their neighboring island, Blefuscu, and they end up engaging in war. They ask the giant Gulliver to help them conquer and enslave Blefuscu but he refuses, resulting in his eventual departure.
The reasoning behind this war was one of "religious principle" as a certain religious debate had come up between the two that was irreparable. This debate was which end of an egg to crack when cooking it—the big end or the small end. The Blefuscuns were "Big Endians" and this angered the Lilliputians to no end.
The resulting war was used to satirize the petty squabbles and drama that would cause countries such as England, France, and Portugal to engage in war.

The cause of the seemingly never-ending conflict between Lilliput and Blefuscu is almost breathtaking in its triviality. The whole thing started over a dispute about the correct end at which to break an egg. The Lilliputians believe that an egg should be broken from the small end. The Blefuscans, however, are so-called "Big Endians," believing with equal conviction that the big end of the egg is the correct place at which to break it.
Gulliver's Travels is a work of satire, and in the pointless, drawn-out conflict between Lilliput and Blefuscu, Swift is poking fun at the ridiculous reasons that countries often give for going to war with each other. During Swift's lifetime, conflicts of one kind or another frequently broke out in Europe. And though the causes of such conflicts were never quite as trivial or absurd as the great egg controversy, Swift nonetheless suggests that the regular wars plaguing Europe in the early 18th century were still wholly unnecessary.