Beware Of The Leaven
The command given by Jesus Christ ("10:1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea; 10:2 And were all baptized unto Moses in the cloud and in the sea; 10:3 And did all eat the same spiritual meat; 10:4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ" 1 Corinthians 10:1-4 KJV; see also 'Before Abraham Was, I AM') to His people after the Exodus:
"12:17 And ye shall observe the feast of unleavened bread; for in this selfsame day have I brought your armies out of the land of Egypt: therefore shall ye observe this day in your generations by an ordinance for ever. 12:18 In the first month, on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even. 12:19 Seven days shall there be no leaven found in your houses: for whosoever eateth that which is leavened, even that soul shall be cut off from the congregation of Israel, whether he be a stranger, or born in the land. 12:20 Ye shall eat nothing leavened; in all your habitations shall ye eat unleavened bread." (Exodus 12:17-20 KJV)
"How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven"
Passover and the Days of Unleavened Bread (also known as the Days of Passover) were commanded by Christ as a single observance because they represent the Sacrifice of the Lamb of God and the required acceptance of His Sacrifice by obeying Him.
"12:13 And the blood [see also Why Blood?] shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.
The applied meaning of the Days of Unleavened Bread are the essence of genuine Christian Living. As stated in the above verses, the Feast of Passover/Unleavened bread memorialized Israel's deliverance from a life of slavery (see Plagues Of The Pharaohs: Past and Future) after the slaying of the Passover lamb, but it ultimately portrayed Christian deliverance from a life of sin and human arrogance under Satan's influence.
"5:6 Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? 5:7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our Passover is sacrificed for us: 5:8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth." (1 Corinthians 5:6-8 KJV)
There was, and is, nothing wrong with eating things containing yeast at other times, but for the purpose of the Days of Unleavened Bread it was used as a symbol of sin. It was also sometimes used as a metaphor for Satanic pride and hypocrisy:
"16:11 How is it that ye do not understand that I spake it not to you concerning bread, that ye should beware of the leaven of the Pharisees and of the Sadducees? [see also The High Priest's Deal With The Devil] 16:12 Then understood they how that he bade them not beware of the leaven of bread, but of the doctrine of the Pharisees and of the Sadducees." (Matthew 16:11-12 KJV)
Fact Finder: How and when are true Christians to observe the genuine Holy Days of the LORD?