The next main aspect of the Passover Feast is the Feast of Unleavened Bread, which is a part of the Passover, as we read in Exodus 12:17:
You shall also observe the Feast of Unleavened Bread, for on this very day I brought your hosts out of the land of Egypt; therefore you shall observe this day throughout your generations as a permanent ordinance.
In Scripture leaven speaks of two things. The first is malice and wickedness as we read in I Corinthians 5:8: "Let us therefore celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth." The second thing that leaven speaks of is both the legalism of the Pharisees and the liberalism of the Sadducees, as the Lord warned in Matthew 16:6: "And Jesus said to them, 'Watch out and beware of the leaven of the Pharisees and Sadducees."'
The Feast of Unleavened Bread is celebrated in remembrance of how Israel came out of Egypt, and it is a revelation of how we keep unwanted leaven out of our lives. As we read in Deuteronomy 16:3:
You shall not eat leavened bread with it;
seven days you shall eat with it unleavened bread,
the bread of affliction (for you came out of the land of Egypt in haste),
in order that you may remember all the days of your life
the day when you came out of the land of Egypt.
The reason Israel ate unleavened bread when they came out of Egypt was because as they fled Egypt "in haste," their bread did not have time to become leavened. This is also how we keep the leaven of malice and wickedness, legalism, or liberalism out of our lives—when we partake of the Passover sacrifice of Christ we must flee the world and worldliness with such haste that leaven does not have time to spring up in us.
One of the basic characteristics of a true Christian life is that it is moving, flowing, and going somewhere. The River of Life was not called a lake, but a "river" because a river is flowing and moving toward a certain destination. As a river gets closer to its destination it usually becomes wider and deeper. Just as water is sometimes used as a metaphor for truth in Scripture, all truth must be moving and growing, becoming both more expansive and deeper for us.
In the very first mention of the Holy Spirit in Scripture (see Genesis 1:2) we see that He is moving. Nowhere does it say that He stopped. Moving is His nature, which is why His acts are often called a "move of the Holy Spirit." As Paul said in Acts 17:28, "for in Him we live and move...." This is how we keep the leaven out of our lives. When any of that which is called leaven begins to creep into a Christian's life, it will almost always be the result of them having stopped growing and moving toward their destiny in Christ.
The children of Israel were required to celebrate this Feast of Unleavened Bread each year with the Passover to remind them of how they left the land of Egypt in such haste that their bread did not have time to become leavened. We too need this constant reminder. Just as water that stops moving will quickly become contaminated, our faith must be ever moving and growing if it is to stay pure.
Just as the greatest opposition to the Lord Jesus when He walked the earth was from the Pharisees and Sadducees, there is a constant attack upon every Christian to turn aside from the River of Life and begin to follow the traditions of men, worshiping what God did in the past rather than what He is doing now. We must honor the great things the Lord has done, and our fathers and mothers in the Lord whom He used to do them through, but we must write our own history.
Christianity is the greatest quest, with the greatest destiny. How could we ever find anything more worthy than the pursuit of God, growing in the knowledge of His ways, and the devotion to do His will?