Then Moses made Israel set out from the Red Sea, and they went into the wilderness of a 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; therefore it was named Marah. And the people grumbled against Moses, saying, “What shall we drink?” And he cried to the Lord, and the Lord showed him a log, and he threw it into the water, and the water became sweet. Exodus 15:22-25
In my Bach Sabbatical studies have been reading a bit recently about Luther’s Hermeneutics of Bible Study.
According to Eric Chafe, Luther believed in the unity of the Old and New Testaments, Christ being the subject of the entire scriptures. He extended this through use of the “analogy of faith” to see in allegory and metaphor for example “comparing the destruction and rebuilding of Jerusalem the roles of Law and Gospel in the individual” 
Luther’s approach differed little from the Puritans in this regard, and I was reminded of this in the passage from Exodus I read today.
Daniel Whedon, in his commentary on the Bible quotes Luther as follows:
Moses causes man to murmur by the terrors of the law, and thus pains him with bitterness, so that he longs for help; and then, when the Holy Spirit comes, at once it [the law] is made sweet. Now this tree of life is the Gospel, the word of the grace, the mercy, and goodness of God. When the Gospel is plunged into the law, and into the knowledge of sin which the law produces, and when it touches a heart in which the law has caused sadness, anxiety, terror, and confusion, it is at once delightful to the taste. 
As a side note, I don’t know whether this precise comment was contained in Bach’s copy of the Calov Bible, which consisted largely of Luther’s comments on scripture, but it seems likely it was and Bach certainly read the biblical passage as he annotated in the margin both before and after. 
English authors, among them Matthew Henry, Spurgeon have drawn attention to the allegory of the cross in this passage. Matthew Henry, as always, is worth quoting:
Some make this tree typical of the cross of Christ, which sweetens the bitter waters of affliction to all the faithful, and enables them to rejoice in tribulation. The Jews’ tradition is that the wood of this tree was itself bitter, yet it sweetened the waters of Marah; the bitterness of Christ’s sufferings and death alters the property of ours. 
For myself I thought more when I read this of applying God’s Word and promises, and later of the Gospel as Luther does.
Whether the cross, or the gospel or the Word of God, it’s good to see how the Old Testament experiences of the people of Israel can still speak to our situation today.
 Chafe, Eric T. (1991). Tonal allegory in the vocal music of J.S. Bach. Berkeley: University of California Press. p.13
 Whedon, Daniel. “Commentary on Exodus 15:1”. “Whedon’s Commentary on the Bible”. http://www.studylight.org/commentaries/whe/view.cgi?bk=1&ch=15. 1874-1909
 Leaver, R. A. (1985). J.S. Bach and scripture: Glosses from the Calov Bible commentary. St. Louis: Concordia Pub. House. p. 71-73
 http://www.biblestudytools.com/commentaries/matthew-henry-complete/exodus/15.html, accessed 18/3/2015