The Israelites had just experienced a miracle of God at the Red Sea but immediately they went back to murmuring and grumbling against God. We never seem to learn, do we?
And when they came to Marah (Marah means "bitter"), they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah. And the people murmured against Moses, saying, What shall we drink? And he cried to the Lord; and the Lord showed him a tree, which when he had cast into the waters, the waters were made sweet: there he made for them a statute and an ordinance, and there he proved them. Ex 15:23-25.
The lack of water to drink signifies the experience of not having any life, only the experience of death. The tree signifies the cross. Gal 3:13; Acts 5:30; 10:39; 1 Pet 2:24. The Israelites were experiencing a type of death when there was no water or the water was bitter.
The take away from this story is this: Bitter thoughts and experiences will surely come your way in life. The question is what do we do with them? What do we do with our pain? We can transmit our pain and bitterness or we can transform it. We can shake our fist at God and become bitter ourselves. We can grumble and complain to God and others. Or we can transform our death experience into life, the resurrection life of Christ.
The remainder is here: http://safeguardyoursoul.com/the-bitterness-of-life-made-sweet/