Israel has just passed through the waters and been made a new creation (15:16 “the people you created”). Like Adam and Eve, food will now be the occasion of their testing. Like Adam and Eve, they will fail. This first-born son is not the true Son who will come and successfully pass all tests.
Where sin abounds, grace super-abounds and this is shown to be true again and again. Without water, Israel faces death but the third day brings a resurrection experience. When they find water, it is bitter. But instead of trusting the LORD who rescued them from a bitter life (Exodus 1:14) their unbelief manifests itself as grumbling. Grumbling seems to be a sin that particularly angers Christ.
Moses takes Israel's complaint to the LORD and is shown “the tree”. The tree turns bitterness into sweetness.
Verse 26 is yet another verse showing two persons called the LORD. The Healer LORD calls Israel to obey the voice of the LORD their God. If they do not obey they are as bad as the Egyptians and therefore subject to the plagues of judgment. Yet the LORD wants to heal his people. Remember this is written before Sinai, before the LORD gives his commands and decrees. So how can Israel pay attention and keep them? In the same way that Abraham did: by faith. Genesis 26:5 shows that Abraham, the man of faith, was counted to be a Law-keeper, despite living before the Law.
Leaving Marah Israel travels to Elim. Moses does not explicitly explain the significance of the twelve springs and seventy palm trees, for that he expects us to read our Bibles. Israel is being reminded of her calling to be a blessing to the nations: the 12 tribes of Israel are to bring life to the '70' nations that make up the world (cf. Genesis 10).
Moving into the Desert of Sin we see Israel's sin, and God's grace, made even more apparent. Despite the miracle at Marah the Israelites grumble as soon as the food supply starts getting low. How often we also grumble that the LORD isn't providing as we think he should, forgetting his sustaining of our life so far, and most of all his provision of his Son. Israel's grumbling, however, seems particularly wicked and will be recalled frequently throughout the Scriptures. The Israelites are rejecting their salvation and their LORD (Psalm 78:22). “If only we had died by the LORD's hand in Egypt.” The slavery is forgotten; their corrupted mind falsely-remembers a glorious time of sitting around eating pots of meat. In contrast, Paul will constantly remind Christians of the true horror of our state before we were saved (e.g. Ephesians).
Israel grumbles against Moses and Aaron, but this chapter stresses repeatedly (read vv.6-12) that their grumbling is against the LORD Christ. Moreover, it is the LORD (not Moses and Aaron) who will provide a solution. In providing food, the LORD is testing Israel, to see whether they will trust his word (v.4). Every morning the food of angels (Psalm 78:24-25) lay scattered over the ground; the double provision on the sixth day (to give them enough food for the Sabbath) meant that the people could not start thinking this was some sort of 'natural' phenomenon. No: six days a week, as the Israelites woke up and walked out of their tent to collect manna, they were being tested: would they depend on God's Word? Would they trust him?
As the Church travelled from Exodus / Creation / Redemption, towards Canaan / Rest / Glory, they were sustained by the bread from heaven, given by the Bread who would one day come from heaven to sustain us by his own body.