Ramban

THEY SHALL BE BAKED WITH LEAVEN. Scripture commanded that these two loaves be leavened, because they are a thanks-offering to G-d for having kept for us the appointed time of the harvest, and a thanks-offering comes with cakes of leavened bread. Perhaps the [usual] prohibition of bringing leavened things as an offering, 2:11. is because leaven alludes to the attribute of justice, for it is called chametz (leaven), just as wine which sours is called ‘chometz yayin v’chometz sheichar’ (vinegar of wine or vinegar of strong drink), the word [chametz] being derived from the expression: the unrighteous ‘v’chomeitz’ (and ruthless man), for [vinegar of wine and vinegar of strong drink] are drinks whose original taste has been “robbed” from them, and they are therefore not suitable for [normal] drinking. Similarly, for my heart yithchameitz’, 73:21. means that it was “in a ferment, and it lost its composure.” Now since the offerings are brought to be acceptable before the Glorious Name, they are therefore not to be brought from objects which possess a strong power to change natural properties of other things [such as leaven], and similarly they are not to be brought from things which are completely sweet). such as honey. [Instead, they are to come] only from things which are blended of different qualities, just as the Rabbis have said with reference to the creation of the world “He combined the attribute of mercy with the attribute of justice, and created the world.” Now on the Festival of Weeks which is the day of the Giving of the Torah [on Mount Sinai], the offering [i.e., the two leavened loaves and the various burnt-offerings, together with their meal-offering which is of unleavened bread, as stated in Verse 18] comes in accordance with the law of the thanks-offering [which was brought with ten leavened cakes and thirty unleavened cakes], for it is the day of the solemn assembly. The student learned in the mysteries of the Cabala will understand. And this is the secret of which our Rabbis have said: “All offerings are destined to cease [in the World to Come], but the thanks-offering will never cease etc.,” for in it are both unleavened and leavened [cakes], comparable to the World to Come [in which the Divine attributes of justice and mercy will forever be in perfect harmony].