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.

3402. 'Anyone touching this man and his wife will surely die' means that Divine Truth and Divine Good were not to be disclosed, insomuch that not even any approach was to be made towards them by faith because of the danger of eternal condemnation if they were profaned. This is clear from the meaning of 'touching this man and his wife' as making an approach towards Divine Truth and Divine Good, represented by Isaac and Rebekah - truth here being mentioned first and good second because the subject is people who belong to the spiritual Church and who are capable of adulterating and indeed of profaning truth, but not good, which is why the expression 'man and wife' (vir et mulier) is used, see 915, 2517; and from the meaning of 'surely dying' as eternal condemnation (which is spiritual death) resulting from profanation, under discussion here.

[2] For information about the Lord in His providence not allowing anyone, because of the danger of eternal condemnation, to enter into goodness and truth, that is, into an acknowledgement of them and affection for them, beyond the point where he is able to remain with them, see above in 3398. For goodness and truth, as stated and shown several times already, are such that when present with a person they retreat to an internal position that is determined by the amount of evil and falsity also present in him. Consequently insofar as the angels present with him from heaven depart, the devil-spirits present with him from hell draw near, and vice versa. To one in whom evil and falsity are present, the removal of good and truth, and consequently of angels, is not apparent, for at that time he is convinced that evil is good, and falsity is truth. This conviction arises from his affection for them and from the resulting delight. And when this is his state he is quite incapable of recognizing that goodness and truth have been removed from him. Goodness and truth - or angels - are said to be removed from a person when he has no affection for them, that is, when he no longer takes any delight in them, and instead his affection is for the things that go with self-love and love of the world, that is, when he takes delight wholly in these.

[3] Having a knowledge of goodness and truth - or holding them in the memory, and testifying to them with the lips - does not constitute possession of goodness and truth, but a heartfelt affection for them does. Nor does anyone possess goodness and truth when his affection for them springs from a desire to gain reputation or wealth by means of them. In his case he is not stirred by any real affection for them but by honour and gain, and he makes such goodness and truth the means for obtaining these. In the next life the goods and truths which such people have known, even proclaimed, are taken away from them, while the self-love and love of the world remain from which their life is derived. From these considerations it may be seen what the situation is with goodness and truth, namely that unless he is able to remain with them to the end of his life no one is allowed to approach goodness and truth with affection and faith, unlike those who actually profane them, for such people are unable to be held back from approaching them.