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.

9298. 'You shall not sacrifice the blood of My sacrifice with anything made with yeast' means that worship of the Lord arising from the Church's truths must not be mingled together with falsities arising from evil. This is clear from the meaning of 'sacrificing' and 'sacrifice' as worship of the Lord, dealt with in 922, 923, 2180, 2805, 2807, 2830, 3519, 6905, 8680, 8936; from the meaning of 'something made with yeast' as something falsified and falsity arising from evil, dealt with in 2342, 7906, 8051, 8058; and from the meaning of 'blood' as truth derived from good, thus the Church's truth, dealt with in 4735, 6978, 7317, 7326, 7846, 7850, 7877, 9127. From these meanings it is evident that 'you shall not sacrifice the blood of My sacrifice with anything made with yeast' means that worship of the Lord arising from the Church's truths must not be mingled together with falsities arising from evil. What falsity arising from evil and falsity not arising from evil are, see 1679, 2408, 4729, 6359, 7272, 8298, 9258.

[2] The reason why truths derived from good, which are the Church's truths, must not be mingled together with falsities arising from evil is that they are in total disagreement. They are opposites, which causes them to clash, as a result of which either good perishes or evil is put to flight; for good comes from heaven, that is, from the Lord by way of heaven, and evil from hell. Truths may, it is true, exist with the evil, and also falsities with the good; but truths present with the evil are not mingled together with falsities arising from evil present with them as long as they reside solely in the memory, serving as the means to evil. As long as this is so the truths are devoid of life. But if the truths are falsified to lend support to evil, which is also brought about by perverse interpretation, they are mingled together. This leads to the profanation of truth, the nature of which, see 1008, 1010, 1059, 1327, 1328, 2051, 2426, 3398, 3399, 3402, 3489, 4601, 6348, 6595, 6959, 6963, 6971, 8394, 8943, 9188.

[3] The fact that accompanying a sacrifice with something made with yeast was forbidden is evident from the following law regarding a minchah, which was burned on the altar together with a sacrifice, the words of that law in Moses being,

Every minchah which you bring to Jehovah shall be made without yeast; no yeast nor any honey shall be used along with the fire-offering you burn to Jehovah. Leviticus 2:11.

From all that has been said it is clear that the profanation of truth is meant by this law. It also explains why the words 'the blood of [My] sacrifice' are used and not simply 'the sacrifice'; for 'the blood' is truth derived from good.