O Israel! trust thou in Jehovah The prophet again resumes the doctrinal point, that the genuine worshippers of God have no cause to fear that he will forsake or frustrate them in the time of need; because he is as much disposed to provide for their safety as he is furnished with power to do it. He proceeds, in the first place, to exhort all the Israelites generally to place their confidence in God; and, secondly, he addresses the house of Aaron in particular; and, thirdly, he sets down all who fear God. For this arrangement there was good cause. God had adopted indiscriminately all the people, to whom also his grace was offered, so that they were bound in common to place their hope in him. In accordance with this Paul says, that the twelve tribes of Israel wait for the promised deliverance, (Acts 26:7) The prophet, therefore, with great propriety first addresses Israel at large. But having in a peculiar manner set apart the Levites for himself, and more especially the priests of the house of Aaron, to take the precedence, and to preside over ecclesiastical matters, he demands more from them than from the common people; not that salvation was promised specially to them, but because it was proper that they who had the exclusive privilege of entering the sanctuary should point out the way to others. As if the prophet had said, Ye sons of Aaron, whom God hath chosen to be the teachers of religion to his people, be ye to others an example of faith, seeing that he hath so highly honored you in permitting you to enter his sanctuary.

O Israel, trust thou in the Lord - This exhortation is founded in a great measure on what had been just said in regard to idols. They had no power. There was no reason why they should be confided in. They could not help in the day of trouble; and as people need a god, and as the idols cannot be to them as gods, the exhortation is addressed to his people to trust him. He would be to them all that was implied in the name God; all that was wanted in a God.
He is their help - The help of those who trust in him. He is able to help them in the time of trouble; he is willing to help them; he will help them. They who put their trust in him will find him a sure and certain help. This is the experience of all who confide in him.
And their shield - Their protector. See Psalm 5:12, note; Psalm 18:2, note; Psalm 33:20, note. Compare Genesis 15:1; Deuteronomy 33:29; Proverbs 30:5.

O Israel - The body of the Jewish people.

O Israel, trust thou in the Lord,.... Or, "the house of Israel hath trusted in the Lord": so the Septuagint, Vulgate Latin, Syriac, Arabic, and Ethiopic versions: the Targum is,
"Israel trusteth in the Word of the Lord;''
in distinction from the Heathens, that trust in their idols. But it is better rendered as an imperative, trust thou; it being an exhortation to Israel to trust in the Lord, in opposition to idols; and may be understood of Israel, literally taken, who were God's chosen covenant people, to whom he had made a revelation of himself, and of his will; and therefore should trust in him, and in no other; and of spiritual Israel, or all the elect of God, and redeemed of the Lamb; every Israelite indeed; every wrestling Jacob, and prevailing Israel; every praying soul; every sensible sinner, Jew or Gentile. It becomes them to trust in the Lord, not in the creature; not in their own strength, wisdom, riches, righteousness, or fleshly privileges; but in the Lord, as the God of nature, providence, and grace; as a promising and covenant keeping God, who is to be trusted with all, and for every thing temporal and spiritual, and at all times.
He is their help and their shield; the help and shield of every true Israelite; of everyone that trusts in the Lord; or,
"your help and your shield, O ye Israelites;''
so Ben Balaam in Aben Ezra reads the words: which are a reason or argument encouraging trust in the Lord, since he is the help of his people; they are helpless in themselves, and vain is the help of man, for there is none in him; there is no help but in the Lord, and he is a present, seasonable, and sufficient help: Jehovah the Father has promised them help, and he is both able and faithful to make it good; he has laid help upon his Son for them; and has set up a throne of grace, where they may come for grace to help them in time of need: Christ has helped them out of the miserable estate they were fallen into by sin; he helps them on in their way to heaven, by his power and grace, and at last brings them thither: the Spirit of God helps them to the things of Christ; to many exceeding great and precious promises; and out of many difficulties, snares, and temptations; and he helps them in prayer under all their infirmities, and makes intercession for them, according to the will of God; and therefore they should trust in the Lord, Father, Son, and Spirit: and who is also "their shield", to protect and defend them from all dangers, evils, and enemies; what a shield is to the body, to secure it from hurt, that to the people of God are the love and favour of God, his power and might, his truth and faithfulness; as likewise Christ, his blood, righteousness, and salvation; and the Spirit, and his grace; see Psalm 5:12, Ephesians 6:16.

It is folly to trust in dead images, but it is wisdom to trust in the living God, for he is a help and a shield to those that trust in him. Wherever there is right fear of God, there may be cheerful faith in him; those who reverence his word, may rely upon it. He is ever found faithful. The greatest need his blessing, and it shall not be denied to the meanest that fear him. God's blessing gives an increase, especially in spiritual blessings. And the Lord is to be praised: his goodness is large, for he has given the earth to the children of men for their use. The souls of the faithful, after they are delivered from the burdens of the flesh, are still praising him; but the dead body cannot praise God: death puts an end to our glorifying him in this world of trial and conflict. Others are dead, and an end is thereby put to their service, therefore we will seek to do the more for God. We will not only do it ourselves, but will engage others to do it; to praise him when we are gone. Lord, thou art the only object for faith and love. Help us to praise thee while living and when dying, that thy name may be the first and last upon our lips: and let the sweet savour of thy name refresh our souls for ever.

The repetitions imply earnestness.

After this confession of Israel there now arises a voice that addresses itself to Israel. The threefold division into Israel, the house of Aaron, and those who fear Jahve is the same as in Psalm 118:2-4. In Ps 135 the "house of Levi" is further added to the house of Aaron. Those who fear Jahve, who also stand in the last passage, are probably the proselytes (in the Acts of the Apostles σεβόμενοι τὸν Θεόν, or merely σεβόμενοι)
(Note: The appellation φοβούμενοι does not however occur, if we do not bring Acts 10:2 in here; but in Latin inscriptions in Orelli-Hentzen No. 2523, and in Auer in the Zeitschrift fr katholische Theologie 1852, S. 80, the proselyte (religionis Judaicae) is called metuens.))
at any rate these are included even if Israel in Psalm 115:9 is meant to signify the laity, for the notion of "those who fear Jahve" extends beyond Israel. The fact that the threefold refrain of the summons does not run, as in Psalm 33:20, our help and shield is He, is to be explained from its being an antiphonal song. In so far, however, as the Psalm supplicates God's protection and help in a campaign the declaration of confident hope, their help and shield is He, may, with Hitzig, be referred to the army that is gone or is going forth. It is the same voice which bids Israel to be of good courage and announces to the people the well-pleased acceptance of the sacrifice with the words "Jahve hath been mindful of us" (זכרנוּ ה, cf. עתּה ידעתּי, Psalm 20:7), perhaps simultaneously with the presentation of the memorial portion (אזכרה) of the meat-offering (Psalm 38:1). The יברך placed at the head is particularized threefold, corresponding to the threefold summons. The special promise of blessing which is added in Psalm 115:14 is an echo of Deuteronomy 1:11, as in 2-Samuel 24:3. The contracted future יסף we take in a consolatory sense; for as an optative it would be too isolated here. In spite of all oppression on the part of the heathen, God will make His people ever more numerous, more capable of offering resistance, and more awe-inspiring.

Their - Who trust in him.