Sola Fide, or "Justification by Faith Alone", is pretty much the linchpin doctrine for most forms of Protestant Christian.
The doctrine finds its origin in the thought of Martin Luther, the German monk who kicked off the "schism-from-the-schism" known as the Protestant Reformation. Sola Fide says, essentially, that through faith-- which seems to be defined as intellectual assent to Christ's crucifixion and resurrection-- Jesus Christ's righteousness is credited to the Christian. Jesus earns righteousness, God the Father pretends the Christian is now righteous. Often, Blessed Augustine of Hippo is used as a pre-Reformation reference point by advocates of Sola Fide. He is quite important, because the Saint is often the only person that Protestants can point ot as supporting their doctrines. If one can prove Blessed Augustine didn't believe or teach Sola Fide, the post-Reformation viewpoint is adrift outside the consensus of the Fathers.
But when it comes to the nature of justification, Blessed Augustine is one with the rest of the Church Fathers, east and west.
"When you shall have been baptized, keep to a good life in the commandments of God so that you may preserve your baptism to the very end. I do not tell you that you will live here without sin, but they are venial sins which this life is never without. Baptism was instituted for all sins. For light sins, without which we cannot live, prayer was instituted. . . . But do not commit those sins on account of which you would have to be separated from the body of Christ. Perish the thought! For those whom you see doing penance have committed crimes, either adultery or some other enormities. That is why they are doing penance. If their sins were light, daily prayer would suffice to blot them out. . . . In the Church, therefore, there are three ways in which sins are forgiven: in baptisms, in prayer, and in the greater humility of penance."
(St. Augustine, Sermon to Catechumens on the Creed 7:15, 8:16)
CHAP. 18.--FAITH WITHOUT GOOD WORKS IS NOT SUFFICIENT FOR SALVATION.
Unintelligent persons, however, with regard to the apostle's statement: "We conclude that a man is justified by faith without the works of the law," have thought him to mean that faith suffices to a man, even if he lead a bad life, and has no good works. Impossible is it that such a character should be deemed "a vessel of election" by the apostle, who, after declaring that "in Christ Jesus neither circumcision availeth anything, nor uncircumcision," adds at once, "but faith which worketh by love." It is such faith which severs God's faithful from unclean demons,--for even these "believe and tremble," as the Apostle James says; but they do not do well. Therefore they possess not the faith by which the just man lives,--the faith which works by love in such wise, that God recompenses it according to its works with eternal life. But inasmuch as we have even our good works from God, from whom likewise comes our faith and our love, therefore the selfsame great teacher of the Gentiles has designated "eternal life" itself as His gracious "gift."
CHAP. 19 [VIII.]--HOW IS ETERNAL LIFE BOTH A REWARD FOR SERVICE AND A FREE GIFT OF GRACE?
And hence there arises no small question, which must be solved by the Lord's gift. If eternal life is rendered to good works, as the Scripture most openly declares: "Then He shall reward every man according to his works:" how can eternal life be a matter of grace, seeing that grace is not rendered to works, but is given gratuitously, as the apostle himself tells us: "To him that worketh is the reward not reckoned of grace, but of debt;" and again: "There is a remnant saved according to the election of grace;" with these words immediately subjoined: "And if of grace, then is it no more of works; otherwise grace is no more grace"? How, then, is eternal life by grace, when it is received from works? Does the apostle perchance not say that eternal life is a grace? Nay, he has so called it, with a clearness which none can possibly gainsay. It requires no acute intellect, but only an attentive reader, to discover this. For after saying, "The wages of sin is death," he at once added, "The grace of God is eternal life through Jesus Christ our Lord."
CHAP. 20.--THE QUESTION ANSWERED. JUSTIFICATION IS GRACE SIMPLY AND ENTIRELY, ETERNAL LIFE IS REWARD AND GRACE.
This question, then, seems to me to be by no means capable of solution, unless we understand that even those good works of ours, which are recompensed with eternal life, belong to the grace of God, because of what is said by the Lord Jesus: "Without me ye can do nothing." And the apostle himself, after saying, "By grace are ye saved through faith; and that not of yourselves, it is the gift of God: not of works, lest any man should boast;" saw, of course, the possibility that men would think from this statement that good works are not necessary to those who believe, but that faith alone suffices for them; and again, the possibility of men's boasting of their good works, as if they were of themselves capable of performing them. To meet, therefore, these opinions on both sides, he immediately added, "For we are His workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them." What is the purport of his saying, "Not of works, lest any man should boast," while commending the grace of God? And then why does he afterwards, when giving a reason for using such words, say, "For we are His workmanship, created in Christ Jesus unto good works"? Why, therefore, does it run, "Not of works, lest any man should boast"? Now, hear and understand. "Not of works" is spoken of the works which you suppose have their origin in yourself alone; but you have to think of works for which God has moulded (that is, has formed and created) you. For of these he says, "We are His workmanship, created in Christ Jesus unto good works." Now he does not here speak of that creation which made us human beings, but of that in reference to which one said who was already in full manhood, "Create in me a clean heart, O God;" concerning which also the apostle says, "Therefore, if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. And all things are of God." We are framed, therefore, that is, formed and created, "in the good works which" we have not ourselves prepared, but "God hath before ordained that we should walk in them."
It follows, then, dearly beloved, beyond all doubt, that as your good life is nothing else than God's grace, so also the eternal life which is the recompense of a good life is the grace of God; moreover it is given gratuitously, even as that is given gratuitously to which it is given. But that to which it is given is solely and simply grace; this therefore is also that which is given to it, because it is its reward;--grace is for grace, as if remuneration for righteousness; in order that it may be true, because it is true, that God "shall reward every man according to his works."
(A Treatise on Grace and Free Will)
Now, if the wicked man were to be saved by fire on account of his faith only, and if this is the way the statement of the blessed Paul should be understood--"But he himself shall be saved, yet so as by fire"--then faith without works would be sufficient to salvation. But then what the apostle James said would be false. And also false would be another statement of the same Paul himself: "Do not err," he says; "neither fornicators, nor idolaters, nor adulterers, nor the unmanly, nor homosexuals, nor thieves, nor the covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the Kingdom of God."
(Enchiridion, Chapter XVIII, paragraph 3).
Blessed Augustine of Hippo, pray unto God for us!