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.

3489. To those within the Church it is not apparent that this is the condition of the Church, that is to say, that they treat with contempt and loathe everything to do with goodness and truth, and also show hostility towards those things, especially towards the Lord Himself. They do indeed attend places of worship, listen to sermons with some kind of reverence while they are there, go to the Holy Supper, and sometimes discuss those things with one another in a seemly way. The evil accordingly do the same things as the good, even exercising common charity or friendship to one another, and as a consequence others do not see in them any contempt for the goods and truths of faith, or therefore any contempt for the Lord, still less any loathing of these, and least of all any hostility towards them. But those very actions are outward forms, by which one person leads another astray, whereas the inward forms existing with members of the Church are completely different and the complete reverse of those outward forms. It is the inward forms which are described here and which are of that nature. The essential nature of these inward forms is presented visually in heaven; for the angels pay no attention to anything else than the things that are internal - to ends in view, that is, to people's intentions and wills, and to their thoughts stemming from these. How different these are from external things becomes clear from members of the Christian world entering the next life, regarding whom see 2121-2126.

[2] Indeed in the next life it is solely in accordance with internal things that people think and speak, for external things have been left behind with the body. There it is evident that however peaceable such people seemed to be in the world they nevertheless hated one another, and hated everything belonging to faith, hating the Lord above all else; for at the mere mention of the Lord's name in their presence in the next life a sphere not only of contempt but also of loathing and hostility towards Him clearly emanates from them and envelops them, including those who to outward appearances spoke about Him with reverence and also preached about Him. It is similar when charity and faith are mentioned. As to their inward form which is disclosed there, these people are such as they would have been while living in the world if external restraints had been released and taken away from them. That is, if they had not in the world feared for their lives and feared the law, and in particular if they had not feared for their reputation on account of the positions they strove and worked for, and of the wealth they desired and avidly sought after, they would on account of their deadly hatred have laid into one another, as their intentions and thought directed them. And without any conscience they would have seized other people's goods and also without any conscience would have butchered them, no matter how utterly innocent their victims may have been. Such is the nature of Christians interiorly at the present day, apart from a few who remain unknown. From all this it is evident what the nature of the Church is essentially.