Romans 3:27-31 "Where then is boasting? It is excluded. By what kind of law? Of works? No, but by a law of faith. 28 For we maintain that a man is justified by faith apart from works of the Law. 29 Or is God the God of Jews only? Is He not the God of Gentiles also? Yes, of Gentiles also, 30 since indeed God who will justify the circumcised by faith and the uncircumcised through faith is one.
The Father has to agree that faith alone is necessary and sufficient to declare the sinner innocent in His sight
Once God has called and convicted the sinner and shown them the awfulness of sin and the awesomeness of Christ, the sinner responds by faith. Justification is a Divine declaration that goes full circle. It is by God the Father, through God the Son and to God the Holy Spirit for the pleasure of God the Father. We must underscore the fact that all that the Triune God accomplished in making salvation a reality was not the by-product of three-wills, but One Divine will. All three persons share in One Divine nature, which means of course that this is the effort of one, unified will. Henceforth we understand from the Bible that there was never any conflict between the three persons of the Trinity.
In short, the doctrine of justification involved the Son expressing His willingness to co-sign a Divine loan issued by the Father and served by the Spirit to sinners that upon their conviction of sin respond in saving faith.
The Dilemma is solved by God in the truth of justification by faith
By planning, purchasing and applying salvation, God is able to solve the dilemma of justifying sinful human beings without violating His Holy Character. This is the whole point of the closing verses in Romans 3:27-31. Romans 3:21-31 spells all of these Divine decisions by God in full detail. Once we understand that only God could solve the otherwise impossible task of justifying guilty sinners, we can truly see why Justification is by faith alone and not by human effort.