This is a great question, all of the things that were a part of the first tabernacle were patterns of the heavenly, and have spiritual significance to us. I would like to share some of the things about unleavened bread, but the first place I would like to start is some verses that show us that God did command that the offerings that were made be made with unleavened bread, and there are others that God told them to offer with leavened bread, I believe this is important too. Even though leavening is often used to show us how something that is bad or wicked can affect the whole, it is also used to show us how something that is good and pure affects the whole, praise God!
Leviticus 2:4 And if thou bring an oblation of a meat offering baken in the oven, it shall be unleavened cakes of fine flour mingled with oil, or unleavened wafers anointed with oil.
Leviticus 7:12 If he offer it for a thanksgiving, then he shall offer with the sacrifice of thanksgiving unleavened cakes mingled with oil, and unleavened wafers anointed with oil, and cakes mingled with oil, of fine flour, fried.
13 Besides the cakes, he shall offer for his offering leavened bread with the sacrifice of thanksgiving of his peace offerings.
Notice that in those two verses they were to offer both unleavened and leavened bread, I love that the leavened bread was offered with the sacrifice of thanksgiving, one of the things that I see in this how when we begin to give thanks to the Lord how it may start off small in our hearts, but it quickly fills our entire being, much like leavening does bread. Praise God!
But I see how for many offerings the Lord commanded them to offer unleavened bread.
Leviticus 8:2 Take Aaron and his sons with him, and the garments, and the anointing oil, and a bullock for the sin offering, and two rams, and a basket of unleavened bread;
Numbers 6:13 And this is the law of the Nazarite, when the days of his separation are fulfilled: he shall be brought unto the door of the tabernacle of the congregation:
14And he shall offer his offering unto the LORD, one he lamb of the first year without blemish for a burnt offering, and one ewe lamb of the first year without blemish for a sin offering, and one ram without blemish for peace offerings,
15 And a basket of unleavened bread, cakes of fine flour mingled with oil, and wafers of unleavened bread anointed with oil, and their meat offering, and their drink offerings.
16 And the priest shall bring them before the LORD, and shall offer his sin offering, and his burnt offering:
17 And he shall offer the ram for a sacrifice of peace offerings unto the LORD, with the basket of unleavened bread: the priest shall offer also his meat offering, and his drink offering.
Leavening is a yeast, it is alive, and there are spores all around us of different kinds that can be, and has been used for this.
People would make a mixture ( could differ depending on the time and culture ) of water, flour, perhaps sugar of some kind, and capture the airborne yeast spores in this mixture, this mixture is known as the " mother " sugars, have to be added to this from time to time to keep it alive and growing, it was from this " mother " that the leavening is added to dough , by taking small amounts from the " mother " and adding to the mother from time to time, it was kept and used for as long as needed. There are people who boast of having the same " mother " for decades, I believe a bakery in San Francisco claims to use the same " mother " for their sour dough bread that they used since the 1800's.
I see the spiritual significance in this, because when the children of Israel were delivered from the Egyptians God commanded them to put away the leavening from out of their houses, and they ate unleavened bread the night of passover, and when they left that night, they took unleavened dough with them.
Exodus 12:15 Seven days shall ye eat unleavened bread; even the first day ye shall put away leaven out of your houses: for whosoever eateth leavened bread from the first day until the seventh day, that soul shall be cut off from Israel.
Look at how the Lord began speaking to them in this chapter,
Exodus 12:1 And the LORD spake unto Moses and Aaron in the land of Egypt saying,
2 This month shall be unto you the beginning of months: it shall be the first month of the year to you.
This was a new beginning for them, it was to them from this point on the first month of the first year. I love this about God, He is the God of new beginnings, when we are in Christ old things are passed away, behold all things are become new.
2 Corinthians 5:17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Remember that the leavening was taken from the air, it was from Egypt. This same place that they had been in bondage, this place that God had to rescue them from and set them free with a mighty and outstretched hand. They were called by His name now, their identity was in the Lord, God was freeing them from all the things in Egypt that tried to cling to them, the things that they couldn't free themselves from, but that God did for them, because He loved them. We can see spiritually that Egypt was the place of their bondage, and God was freeing them from all of these things that had power over them, so that they could serve Him in liberty and in truth. They didn't need to have those things from Egypt still clinging to them, those things that only worked death, and bondage to them, they were a new creation in Him, and this is where their identity should be.
We know that even after the Lord delivered them out of Egypt that there was some that were unthankful, they become murmurers and complainers, and in their hearts they turned back to Egypt desiring the way their life was, they desired and went back in their hearts after wicked things, even the things that God set them free from.( Numbers chapter 11)
They even despised what the Lord provided for them to eat, what they called manna, their hearts were hardened, and they didn't consider in their hearts all of the great things that God had done for them.
And many of them died in the wilderness because of this, and these things are written for our benefit so that we wouldn't be like them.
Acts 7: 37 This is that Moses, which said unto the children of Israel, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear.
38 This is he, that was in the church in the wilderness with the angel which spake to him in the mount Sina, and with our fathers: who received the lively oracles to give unto us:
39 To whom our fathers would not obey, but thrust him from them, and in their hearts turned back again into Egypt,
40 Saying unto Aaron, Make us gods to go before us: for as for this Moses, which brought us out of the land of Egypt, we wot not what is become of him.
1 Corinthians 10:5 But with many of them God was not well pleased: for they were overthrown in the wilderness.
6 Now these things were our examples, to the intent we should not lust after evil things, as they also lusted.
7 Neither be ye idolaters, as were some of them; as it is written, The people sat down to eat and drink, and rose up to play.
8 Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.
9 Neither let us tempt Christ, as some of them also tempted, and were destroyed of serpents.
10 Neither murmur ye, as some of them also murmured, and were destroyed of the destroyer.
11 Now all these things happened unto them for examples: and they are written for our admonition, upon whom the ends of the world are come.
12 Wherefore let him that thinketh he standeth take heed lest he fall.
13 There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.
So spiritually they didn't need to keep eating from the things that were of Egypt, because it only worked death to them, as all sin does when it is finished. They needed to be nourished from the Lord, and what He provided them to eat was the same thing that many were complaining about. I doubt that they become so unthankful and blind in one day, they were probably glad to be out of Egypt at the first, but something happened in their hearts that changed that. Just like leavening, these things that start out small can grow in us if we are not purged by the Lord, these feelings of resentment, selfishness, hypocrisy, insincerity, hate, malice, can grow in us if we choose to eat of it, instead of what God has given us. Jesus Christ is the True Bread from Heaven, the Manna that God has given to us.
John 6:47 Verily, verily, I say unto you, He that believeth on me hath everlasting life.
48 I am that bread of life.
49 Your fathers did eat manna in the wilderness, and are dead.
50 This is the bread which cometh down from heaven, that a man may eat thereof, and not die.
51 I am the living bread which came down from heaven: if any man eat of this bread, he shall live for ever: and the bread that I will give is my flesh, which I will give for the life of the world.
Hypocrisy is one of the things in God's word that leaven is compared too, and Jesus told us to beware of this.
Luke 12:1 In the mean time, when there were gathered together an innumerable multitude of people, insomuch that they trode one upon another, he began to say unto his disciples first of all, Beware ye of the leaven of the Pharisees, which is hypocrisy.
2 For there is nothing covered, that shall not be revealed; neither hid, that shall not be known.