Can you explain Isaiah 26:3? What is "perfect peace"?
Let's start in Isaiah 25 for context:
Isaiah 25:9-12 And it shall be said in that day, Lo, this is our God; we have waited for him, and he will save us: this is the LORD; we have waited for him, we will be glad and rejoice in his salvation. 10 For in this mountain shall the hand of the LORD rest, and Moab shall be trodden down under him, even as straw is trodden down for the dunghill. 11 And he shall spread forth his hands in the midst of them, as he that swimmeth spreadeth forth his hands to swim: and he shall bring down their pride together with the spoils of their hands. 12 And the fortress of the high fort of thy walls shall he bring down, lay low, and bring to the ground, even to the dust.
Isaiah 26:1-4 In that day shall this song be sung in the land of Judah; We have a strong city; salvation will God appoint for walls and bulwarks. 2 Open ye the gates, that the righteous nation which keepeth the truth may enter in. 3 Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee. 4 Trust ye in the LORD for ever: for in the LORD JEHOVAH is everlasting strength...
In context Isaiah is prophesying of the day when God will save Israel by the redeemer (Jesus Christ) as he has promised.
He is speaking of Christ’s second advent when he will overcome Israel’s enemies and restore the kingdom of Israel as the kingdom of God.
The angel Gabriel told Mary:
Luke 1:32,33 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
This is the time spoken of in Isaiah 65:
Isaiah 65:18,19 But be ye glad and rejoice for ever in that which I create: for, behold, I create Jerusalem a rejoicing, and her people a joy. 19 And I will rejoice in Jerusalem, and joy in my people: and the voice of weeping shall be no more heard in her, nor the voice of crying.
At his return Jesus Christ will raise the faithful of all ages from the dead and they will reign with him over the mortal population of the world for 1000 years. The people of Israel who are alive at his return will recognise him as their Messiah and be reconciled to God. They will be restored to the land and a new sanctuary and Temple will be built as described in Ezekiel 40 to 48.
This restoration is the hope of the Gospel which has been preached to all nations since the first century AD. This hope enables those who believe and obey the Gospel to live their lives with a perfect inner peace with God. It doesn’t mean that their lives will be peaceful in this world.
Jesus described it to his disciples
John 16:33 These things I have spoken unto you, that in me ye might have peace. In the world ye shall have tribulation: but be of good cheer; I have overcome the world.
John 14:27 Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid.
Jesus is called the Prince of peace.
Isaiah 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Paul writes to the Romans:
Romans 5:1 Therefore being justified by faith, we have peace with God through our Lord Jesus Christ
Being at peace with God in our lives is the perfect peace prophesied by Isaiah.
As Paul writes to the Ephesians:
Ephesians 2:14-17 For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us; 15 Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace; 16 And that he might reconcile both unto God in one body by the cross, having slain the enmity thereby: 17 And came and preached peace to you which were afar off, and to them that were nigh.
And to the Philippians:
Philippians 4:7 And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
And to the Colossians:
Colossians 3:15 And let the peace of God rule in your hearts, to the which also ye are called in one body; and be ye thankful.
The same idea appears in the Psalms
Psalms 56:4 In God I will praise his word, in God I have put my trust; I will not fear what flesh can do unto me.
Psalms 118:6 The LORD is on my side; I will not fear: what can man do unto me?
And is picked up by the writer to the Hebrews:
Hebrews 13:6 So that we may boldly say, The Lord is my helper, and I will not fear what man shall do unto me.
When we believe the Gospel and are baptised into Jesus Christ we can have complete confidence in God’s promises; knowing that if we remain faithful, nothing can separate us from the love of God.
Romans 8:38,39 For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, 39 Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.
I hope this helps.