John Gill Commentary Isaiah 26:3

"Thou wilt keep [him] in perfect peace, [whose] mind [is] stayed [on thee]; because he trusteth in thee." — Isaiah 26:3 (ASV)

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 sense the word "perfect" is used, in opposition to a false and imaginary one; and it will end in perfect peace in heaven: moreover, the word "perfect" is not in the Hebrew text, it is there "peace, peace"; which is doubled to denote the certainty of it, the enjoyment of it, and the constancy and continuance of it; and as expressive of all sorts of peace, which God grants unto his people, and keeps for them, and them in; as peace with God and peace with men, peace outward and peace inward, peace here and peace hereafter; and particularly it denotes the abundance of peace that believers will have in the kingdom of Christ in the latter day; see (Psalms 72:7) (Isaiah 9:7) :

[whose] mind [is] stayed [on thee] ;
or "fixed" on the love of God, rooted and grounded in that, and firmly persuaded of interest in it, and that nothing can separate from it; on the covenant and promises of God, which are firm and sure; and on the faithfulness and power of God to make them good, and perform them; and on Christ the Son of God, and Saviour of men; upon him as a Saviour, laying the whole stress of their salvation on him; upon his righteousness, for their justification; upon his blood and sacrifice, for atonement, pardon, and cleansing; on his fulness, for the supply of their wants; on his person, for their acceptance with God; and on his power, for their protection and preservation; see (Isaiah 10:20) (50:10) (Psalms 112:7Psalms 112:8) :

because he trusteth in thee ;
not in the creature, nor in any creature enjoyment, nor in their riches, nor in their righteousness, nor in their own hearts, nor in any carnal privileges: only in the Lord, as exhorted to in the next verse (Isaiah 26:4) ; in the Word of the Lord, as the Targum, that is, in Christ.