Isaiah 26:3 (BSB)

You will keep in perfect peace the steadfast of mind, because he trusts in You.

From Isaiah 26. Also in the ESV.

Commentary on Isaiah 26:3

• John Calvin (Reformed), Calvin's Commentaries on Isaiah 26:3: 3. The thought is fixed; thou wilt keep peace, peace. {Bogus footnote} As the Hebrew word יצר (Yĕtzĕr) signifies both “imagination” or “creature,” and “thought,” some render it, “By a settled foundation thou wilt keep peace;” as if the Prophet meant, that when men, amidst the convulsions of the world, continue to rest firmly on God, they will always be safe.
• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Isaiah 26:1-4: To the prophecies of gospel grace very fitly is a song annexed, in which we may give God the glory and take to ourselves the comfort of that grace: In that day, the gospel day, which the day of the victories and enlargements of the Old Testament church was typical of (to some of which perhaps this has a primary reference), in that day this...
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Isaiah 26:3: Thou wilt keep him in perfect peace,.... Peace with God in Christ through his blood, in a way of believing, and as the fruit and effect of his righteousness being received by faith; this is not always felt, received, and enjoyed in the soul; yet the foundation of it always is, and is perfect; and besides, this peace is true, real, and solid; in which...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Isaiah 26:3: mind . . . stayed-- (Psa 112:7-8). Jesus can create "perfect peace" within thy mind, though storms of trial rage without (Isa 57:19; Mar 4:39); as a city kept securely by a strong garrison within, though besieged without (so Phi 4:7).