Israel saw the great work that the LORD did against the Egyptians. So the people feared the LORD and believed in the LORD and in his servant Moses. (14.31)

The Israelites just came through the plentiful waters of the Red sea.  They saw the signs and wonders of the Lord, were delivered from servitude, and believed.  In the same way, at the beginning of our spiritual life and at other stages, the Lord in his mercy delivers us from servitude to the world through the water, that is, through spiritual consolations which are so plentiful and visible as to truly be considered "signs and wonders," and we believe in the Lord.  In the overabundance of his mercies, which are new every day (Lamentations 3.23), we may come to see this as the state of things, paying no attention to the morrow, and seeking to make a dwelling for ourselves like Peter on the mountain of Christ's transfiguration, "for it is good for us to be here." (Matthew 17.4)

But we are pilgrims and strangers on this earth, on our way to the promised land, and so like Christ are "led by the Spirit into the wilderness to be tempted by the devil." (Matthew 4.1)

Then Moses ordered Israel to set out from the Red Sea, and they went into the wilderness of Shur. They went three days in the wilderness and found no water.  When they came to Marah, they could not drink the water of Marah because it was bitter. That is why it was called Marah. (15.22-23)

The wilderness here represents the beginnings of spiritual desolation.  At first, this lack of water is frightening and unsettling.  God seems distant and we may think we are to blame.  And so the first water we come upon, our prior practice of prayer, now seems bitter and unpleasant.

He cried out to the LORD; and the LORD showed him a piece of wood; he threw it into the water, and the water became sweet. There the LORD made for them a statute and an ordinance and there he put them to the test. He said, ‘If you will listen carefully to the voice of the LORD your God, and do what is right in his sight, and give heed to his commandments and keep all his statutes, I will not bring upon you any of the diseases that I brought upon the Egyptians; for I am the LORD who heals you.’ (15.25-26)

The Lord is merciful by nature and cannot be anything but merciful.  Our minds have a difficult time reconciling his justice with his mercy, but with God they are one.  And so the diseases sent upon the Egyptians were acts of mercy toward them (and acts of justice to the enslaved Israelites), increasing in severity, because they would not listen to the Lord's voice or heed his commands which are written on every heart.  He tried to awaken them.

In the same way, if we listen carefully to the voice of the Lord and follow his way, we will not doubt that he is with us, and he will not visit us with spiritual diseases to correct us:  "And since they did not see fit to acknowledge God, God gave them up to a debased mind and to things that should not be done." (Romans 1.28)

So Moses and Aaron said to all the Israelites, ‘In the evening you shall know that it was the LORD who brought you out of the land of Egypt, and in the morning you shall see the glory of the LORD, because he has heard your complaining against the LORD. For what are we, that you complain against us?’ (16.6-7)

The Israelites complained of their hunger and they longed for their fleshpots.  We, too, may sometimes find ourselves tempted to long for our fleshpots, for those worldly allurements which once so captivated us.  But that longing arises when we stop paying heed to the knowledge that God provides our true needs; He alone can fulfill our hunger, even if it isn't our bodily hunger, for he tells us "Do not work for the food that perishes, but for the food that endures for eternal life, which the Son of Man will give you." (John 6.27)

The first evening only, it seems, quails came and covered the camp.  And every morning, manna.  In the evening you must remember that God has preserved you this day, and recall the wonders he has wrought in your life, so you do not grumble against him for "the sun must not go down on your anger.  Do not give the devil a chance to work on you" (Ephesians 4.26-27) through doubt, complacency, ingratitude or forgetfulness.

In the morning we rise with expectancy and eagerly seek "the bread of God which comes down from heaven and gives life to the world," that is, Christ himself, the word who is the bread of life.  "Whoever comes to me will never be hungry, and whoever believes in me will never be thirsty." (John 6.35)

Note that we must rise and seek out our manna, for he says "whoever comes to me."  But herein lies a test for us ("In this way I will test them" (Exodus 16.4)).  Do we seek to gather more than one day's worth, and thus disobey the Lord?  The Israelites were commanded to gather one day's worth only, so that each day they may trust the Lord anew.  Those who disobeyed discovered the manna became foul.

In the same way we, too, are commanded to seek God's word every day: "give us this day our daily bread" (Matthew 6.11).   We disobey him when we pray on one day and think our prayers suffice for two days, worse still - all week!  Daily prayer in full trust is that "food which endures for eternal life." (John 6.27)  Insincere prayer uttered out of obligation only, not intended to be renewed each morning, becomes spoiled manna, "treasure stored up on earth which rusts." (Matthew 6.19)

"His mercies are new every morning" (Lamentations 3.23) and so must be sought every morning.  There is something pernicious about gathering for more than one day: we presume that we will be alive.  "Come now, you who say, 'Today or tomorrow we will go to such and such a town and spend a year there, doing business and making money.'  Yet you do not even know what tomorrow will bring.  What is your life?  For you are a mist that appears for a little while and then vanishes." (James 4.13-14)  "Do not worry about tomorrow.  Today's trouble is enough for today." (Matthew 6.34)