And to the one who does not work but believes in him who justifies the ungodly, his faith is counted as righteousness, Romans 4:5. 

Another of the five cries of the reformers in the fifteenth and sixteenth century was the all - important stance of ‘sola fide’ or ‘faith alone.’ We may ask, why this was so important?
It was as vital in history as it is today. The reason being is that man centred religion always adds something to the biblical gospel in an attempt to appease God for forgiveness of sin in order to have a clean slate.

In Martin Luther’s day, one of the wicked additions the Roman Church used, was the paying of indulgences to the church. This was purely a fund raising scheme for the church and its clergy. It was a scam, which convinced mostly poor people, that for a fee they could have their sins forgiven. Also, “for a fee, bereaved relatives could get a deceased loved one out of Purgatory. At the right price, they could also save up for their own future sins.” [] These fees were called indulgences. Faith alone in Christ alone was derailed hence the reformation claimed back the biblical and foundational cry of ‘sola fide’ for the true church.

Again read James Mongomery Boice: 
The Reformers never tired of saying that justification is by grace alone through faith alone because of Christ alone. When put into theological shorthand the doctrine was expressed as justification by faith alone, the article by which the church stands or falls, according to Martin Luther. The Reformers called justification by faith Christianity's material principle, because it involves the very matter or substance of what a person must understand and believe to be saved. Justification is a declaration of God based on the work of Christ. It flows from God's grace and it comes to the individual not by anything he or she might do but by "faith alone" (sola fide). We may state the full doctrine as: Justification is the act of God by which he declares sinners to be righteous because of Christ alone, by grace alone, through faith alone.