John Calvin Commentary Exodus 15:25

"An he cried unto Jehovah; And Jehovah showed him a tree, and he cast it into the waters, and the waters were made sweet. There he made for them a statute and an ordinance, and there he proved them;" — Exodus 15:25 (ASV)

And he cried. From this we gather that Moses alone prayed properly when the people tumultuously rose against him, and that those who were not worthy of the common air itself were abundantly supplied with sweet water. In this, the inestimable mercy of God shone forth, who condescended to change the nature of the water to supply such wicked, rebellious, and ungrateful people. He could have given them sweet water to drink from the beginning, but He wished by means of the bitter water to highlight the bitterness that lurked in their hearts. He also could have corrected the bitterness in the waters by His will alone, so that they would have become sweet spontaneously.

It is not certain why He preferred to use the tree, unless it was to rebuke their foolish impiety by showing that He has many remedies in His power for every affliction. A question also arises concerning the tree: whether it inherently possessed the quality that it demonstrated there. Although probable arguments may be presented on both sides, I am more inclined to the opinion that there was indeed a natural power hidden in the tree, and yet the taste of the water was miraculously corrected. This is because it would have been difficult so quickly to collect enough of the tree to purify a river, for 600,000 men, together with their wives, children, and cattle, would not have been satisfied with a small stream. However, I am led by a significant reason to think that this property already existed in the tree, because it is clear that a specific type of tree was shown to Moses. Yet, that does not prevent us from believing that a greater effectiveness than usual was imparted to it, so that the waters would be immediately sweetened when it was put into them.

What follows in the second part of the verse can be understood in two ways: namely, either that although God had ordained a statute there, He was nevertheless tempted by the people; or, because God was tempted by the people, He therefore had ordained the statute. If the first meaning is preferred, their crime will be magnified by the comparison, for the impiety of the people was even worse because, although they were taught by the voice of God, yet in that very same place they gave free rein to their rebellious spirit. However, I prefer the latter meaning: that is, that God chastised the sin of the people by whom He had been tempted.

It was, in fact, a kind of testing of God, because they not only doubtfully asked who would give them water, but in these words also revealed their despair. But because in the same context it is said, there he made for them a statute, and there he tempted (or proved) them, God appears to be the subject in both clauses, while it is stated of the people that they received the ordinance and were tested. Therefore, the meaning will be that after God had tested His people through the lack of water, He simultaneously admonished them by His word, so that in the future they should submit themselves more teachably and obediently to His commands.