And he cried unto the LORD; and the LORD shewed 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 {n} he proved them,

(n) That is, God, or Moses in God's name.

Wesley's Notes

15:25 And he cried unto the Lord - It is the greatest relief of the cares of magistrates and ministers, when those under their charge make them uneasy, that they may have recourse to God by prayer; he is the guide of the church's guides, and to the chief shepherd, the under shepherds must on all occasions apply themselves: And the Lord directed Moses to a tree, which he cast into the waters, and they were made sweet - Some think this wood had a peculiar virtue in it for this purpose, because it is said, God shewed him the tree. God is to be acknowledged, not only in the creating things useful for man, but in discovering their usefulness. But perhaps this was only a sign, and not a means of the cure, no more than the brazen serpent. There he made a statute and an ordinance, and there he proved them - That is, there he put them upon trial, admitted them as probationers for his favour. In short he tells them, Exo 15:26, what he expected from them, and that was, in one word, obedience. They must diligently hearken to his voice, and give ear to his commandments, and must take care, in every thing, to do that which was right in God's sight, and to keep all his statutes. Then I will put none of these diseases upon thee - That is, I will not bring upon thee any of the plagues of Egypt. This intimates, that if they were disobedient, the plagues which they had seen inflicted on their enemies should be brought on them. But if thou wilt be obedient, thou shalt be safe, the threatening is implied, but the promise is expressed, I am the Lord that healeth thee - And will take care of thee wherever thou goest.

Scofield Reference Notes

[1] when he had cast into the waters

These bitter waters were in the very path of the Lord's leading, and stand for the trials of God's people, which are educatory and not punitive. The "tree" is the cross Gal 3:13 which became sweet to Christ as the expression of the Father's will Jn 18:11. When our Marahs are Song taken we cast the "tree" into the waters. Rom 5:3,4.

Jamieson-Fausset-Brown Bible Commentary

25. the Lord showed him a tree, which when he had cast into the waters, the waters were made sweet-Some travellers have pronounced this to be the Elvah of the Arabs-a shrub in form and flower resembling our hawthorn; others, the berries of the Ghurkhud-a bush found growing around all brackish fountains. But neither of these shrubs are known by the natives to possess such natural virtues. It is far more likely that God miraculously endowed some tree with the property of purifying the bitter water-a tree employed as the medium, but the sweetening was not dependent upon the nature or quality of the tree, but the power of God (compare Joh 9:6). And hence the "statute and ordinance" that followed, which would have been singularly inopportune if no miracle had been wrought.

and there he proved them-God now brought the Israelites into circumstances which would put their faith and obedience to the test (compare Ge 22:1).

Matthew Henry's Concise Commentary

15:22-27 In the wilderness of Shur the Israelites had no water. At Marah they had water, but it was bitter; so that they could not drink it. God can make bitter to us that from which we promise ourselves most, and often does so in the wilderness of this world, that our wants, and disappointments in the creature, may drive us to the Creator, in whose favour alone true comfort is to be had. In this distress the people fretted, and quarrelled with Moses. Hypocrites may show high affections, and appear earnest in religious exercises, but in the time of temptation they fall away. Even true believers, in seasons of sharp trial, will be tempted to fret, distrust, and murmur. But in every trial we should cast our care upon the Lord, and pour out our hearts before him. We shall then find that a submissive will, a peaceful conscience, and the comforts of the Holy Ghost, will render the bitterest trial tolerable, yea, pleasant. Moses did what the people had neglected to do; he cried unto the Lord. And God provided graciously for them. He directed Moses to a tree which he cast into the waters, when, at once, they were made sweet. 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. But a rebellious Israelite shall fare no better than a rebellious Egyptian. The threatening is implied only, the promise is expressed. God is the great Physician. If we are kept well, it is he that keeps us; if we are made well, it is he that recovers us. He is our life and the length of our days. Let us not forget that we are kept from destruction, and delivered from our enemies, to be the Lord's servants. At Elim they had good water, and enough of it. Though God may, for a time, order his people to encamp by the bitter waters of Marah, that shall not always be their lot. Let us not faint at tribulations.