What does Isaiah 26:3 mean?

This is one of the most famous and most loved verses in all of Isaiah, and with good reason. However, It must be taken in the context of what Isaiah is writing. This song to the Lord is specifically regarding a song of praise to be sung in Judah during the Lord's reign as king on the earth (Isaiah 26:1). The nations who hold to faith in the Lord may enter God's eternal and secure city. Isaiah is describing a "city," ruled by God, in contrast to the worldly "city" of prior verses (Psalm 25:2). Human righteousness will not be enough for a nation to be welcomed into the city. It is reserved for those who trust in the Lord (Isaiah 26:2).

Now Isaiah moves from the nation to individual people living long before the arrival of Messiah and His city. Isaiah says to those in the present that "perfect peace" is possible here and now. The Lord provides this to those whose minds are focused on the Lord through trusting in Him.

Perfect peace is not a state without any anxiety at all, or one where life is always easy and perfect. The prophet is describing internal peace. This would be a feeling of confident contentment independent of circumstances. It is a mind focused on trusting the Lord to provide all that is needed in the present, the future, and for salvation.

Why, then do many of the Lord's people live in anxiety, fear, and conflict? Isaiah would answer that our minds are not focused on the Lord. Paul describes what it means to keep our minds directed to God in Philippians 4:4–9. He describes God's promise to those in Christ who trust Him: "The peace of God, which surpasses all understanding, will guard your hearts and your minds in Christ Jesus" (Philippians 4:7).

To live in such perfect peace is available to every Christian. But it can be a hard-fought battle to rest in that faith in the Lord. That battle may be harder for some than others. But it is possible to rest in that hope of the Lord, even if some days are hard.