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.

593. 'Whom I have created from upon the face ¹ of the ground' means mankind among the descendants of the Most Ancient Church. This is clear not only from the use of the word 'created' in the phrase 'man whom He created', that is, whom He regenerated, and subsequently from the use of the word 'made' in the phrase 'whom He made', that is, whom He perfected or regenerated to the point of his becoming celestial; but also from the use of the expression 'from upon the face ¹ of the ground'. The ground is where the Church is, as shown already. It is additionally clear from the fact that the subject is the people who immersed doctrinal matters concerning faith in their own evil desires. Those however who had no doctrine of faith were unable to act in this fashion. For people outside of the Church have no knowledge of truth and good, and those who do not have that knowledge can have a type of innocence even when they say or do something that is contrary to the truths and goods of faith. For they can be inspired with zeal for the system of worship which they have been brought up in since early childhood, and which they therefore suppose to be true and good. But in the case of people who do possess a doctrine of faith the matter is altogether different, for they are capable of mixing truths with falsities, and sacred things with profane. For this reason their lot in the next life is far worse than that of those called gentiles, who in the Lord's Divine mercy will be considered later on.

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