By faith alone, from the Latin Sola Fide, is one of the five Solas of the 16th century Protestant Reformation. This was Martin Luther's cry in reference to Justification, our "right-standing" before God. Justification, he declared, is by faith alone, not faith plus anything else. Luther believed this was a critical doctrine and regarded it as the truth by which the church would either stand or fall.
Rooted in "sola gratia"
"The doctrine of free justification by faith alone, which became the storm center of so much controversy during the Reformation period, is so often regarded as the heart of the Reformers’ theology but this is not accurate. The truth is that their thinking was really centered upon the contention of Paul, echoed by Augustine and others, that the sinner’s entire salvation is by free and sovereign grace only, and that the doctrine of justification by faith was important to them because it safeguarded the principle of sovereign grace. The sovereignty of grace found expression in their thinking at a more profound level still in the doctrine of monergistic regeneration." - J. I. Packer