Romans 1:16,17 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek. For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.
Galatians 3:7-14 Know ye therefore that they which are of faith, the same are the children of Abraham. And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed. So then they which be of faith are blessed with faithful Abraham. For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them. But that no man is justified by the law in the sight of God, it is evident: for, The just shall live by faith. And the law is not of faith: but, The man that doeth them shall live in them. Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree: That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.
Hebrews 10:38 Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him.
What is faith then?
Hebrews 11:1 Now faith is the substance of things hoped for, the evidence of things not seen.
Nobody alive has seen Yeshua, yet we believe He is God. That is faith. Our faith is built upon belief in the Bible. Our belief in Christ, whom the Bible is all about Revelation 19:10, is the evidence of what we can't see. Faith is what we cling to because it is promised to be connected to our salvation, which is what we all hope for. So the just are those who accept Christ's sacrifice for our sins and we repent for them. That's what He died on the cross for. They aren't just in themselves, but only through believing in Christ.