Justification by Faith is by promise. God promised to Justify a particular seed by Faith, its a Gospel Promise. Its the seed of Abraham Gal 3:7-9
7 Know ye therefore that they which are of faith, the same are the children/seed of Abraham.
8 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.
9 So then they which be of faith are blessed with faithful Abraham.
The blessing of which all nations be blessed is " God Justifying through Faith" So Justification by Faith is a promised blessing to the seed of Abraham, his Spiritual seed.
Again the promise of God Rom 3:30
Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.
The word shall denotes Gods promise to do something.
Justification by Faith depends on Gods Promise, and therefore sure to all the seed Rom 4:16
Therefore it is of faith, that it might be by grace; to the end the promise might be sure to all the seed; not to that only which is of the law, but to that also which is of the faith of Abraham; who is the father of us all,