God is the Savior of His people in absolutely every situation. They are His kingdom, and He is willing and capable of successfully defending His own. That's the theme of this song. It's a theme calculated to renew the courage of Israel's warriors, of their loved ones, and of the entire nation in the face of war.
With that in mind, we should be reminded of Jehoshaphat's experience in his war against the armies of Ammon, Moab, and Mount Seir. In that instance, the Spirit of the Lord came upon Jahaziel, and he prophesied to them the day before they were to go out to face the enemy. He said to them, "Listen, all you of Judah and you inhabitants of Jerusalem, and you, King Jehoshaphat! Thus says the Lord to you: 'Do not be afraid nor dismayed because of this great multitude, for the battle is not yours, but God's. Tomorrow go down against them. They will surely come up by the Ascent of Ziz, and you will find them at the end of the brook before the Wilderness of Jeruel. You will not need to fight in this battle. Position yourselves, stand still and see the salvation of the Lord Who is with you, O Judah and Jerusalem! Do not fear or be dismayed; tomorrow go out against them, for the Lord is with you!'" It was God's kingdom, and He intended to defend it in a way that would reveal His power and glory.
Needless to say, the result was victory for God's army. And the lesson to be learned was simple and straightforward - You are GOD'S people, therefore HE will fight your battles! So the singer says, "God is our refuge and strength, a very present help in trouble!" The people were thus reminded, each time they used this song in worship, that in every battle they were to trust not themselves, but Jehovah their covenant Lord.
And it could never matter how strong the enemy or how great the odds against them. In all cases they could still sing, "Therefore we WILL NOT FEAR, even though the earth be removed, and though the mountains be carried into the midst of the sea; though the waters roar and be troubled, though the mountains shake with its swelling!"
The reason for this trust in Jehovah is revealed in the promise of verses 4 and 5, where the people are told that "There is a river whose streams make glad the city of God. God is in the midst of her, she shall not be moved." God manifests His presence there, it is His special city, and it is therefore impregnable.
The promise is for us as well. We should be reminded of it in the prayer our Lord taught His disciples, a prayer in which He instructed them to say, "For Thine is the kingdom, and the power, and the glory forever." The people of God are His kingdom, His property, His delightful possession, and He will always defend them - just because they are His!
There's a time coming when all battles will cease, when God's rule on earth will be complete and glorious. The psalm is about that glorious future as well. The people sing, "He makes wars to cease to the end of the earth; He breaks the bow and cuts the spear in two; He burns the chariot in the fire!"
God is just telling them in this song that not only Israel, but all the universe is His, and there's a time coming when all evil shall be done away, when His rule alone will be seen not only in Israel, but in all His creation!
With that glorious end in view, and with the God Who brings it about doing the speaking, the conclusion is appropriate. It's an exhortation. He sys to them, "be still, and know that I am God; I will be exalted among the nations, I will be exalted in the earth!"
The application to those who sang this song in the temple is obvious - "The Lord of hosts is with us; the God of Jacob is our refuge!" They are to trust Him! But these things are still true. Only today we are closer to that glorious day when our Lord shall return to cleanse and renew His creation. In view of that hope, then, we too should be ready for all the battles of this world, whether they are against our own flesh, the world, or the Devil himself, and "the battle is not yours, but the Lord's"