For something to be "unleavened" means that it's been made without yeast. Since yeast is what makes bread rise and take on its airy texture, unleavened bread is relatively flat, dense and hard. The idea that unleavened bread is holy appears in both the Old and New Testaments and is still part of both Jewish and Christian traditions.

In the Word, bread represents the desire for what is good -- which gives us spiritual energy, just as bread gives us natural energy. Yeast, meanwhile, represents falsity -- so that bread made without yeast is a pure desire for good, free of falsity. This also makes sense because air represents our capacity to understand what's true -- which is the same capacity we use to embrace ideas that are false. Yeast causes bread to rise by producing gases as it ferments, thus adding "bad" air to the bread.

1328. 'And from there Jehovah scattered them over the face ¹ of the whole Earth' means that internal worship ceased to exist. This becomes clear from the meaning of 'scattering' as being dispersed. In the proximate sense those who wanted to build the city of Babel are meant by 'being scattered over the face ¹ of the whole earth'. But because they are people who deprive others of all knowledge of truth, as has been stated, these words at the same time mean the loss of internal worship; for one is the consequence of the other. Here the consequence is meant because this is the third time that this statement is made. That the first Ancient Church had been deprived of cognitions of truth and good is clear from the consideration that the nations which constituted that Ancient Church became for the most part idolatrous, though they still possessed some form of external worship. The lot of idolaters outside the Church is far better than that of idolaters inside it. The former are external idolaters, but the latter are internal. The fact that the lot of the former is better is clear from what the Lord says in Luke 13:23, 28-30; Matthew 8:11-12. This then is the reason why the state of this Ancient Church was altered.

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