Why is Justification by Faith alone so Important?

 I bought Schreiners book Faith alone. In it he says, "How important is 'Faith alone' - the doctrine of justification? I am not arguing that sola fide is the gospel, though I believe it is one element ot entailment of the gospel. Those who reject the motto aren't necessarily proclaiming a different gospel."

Well Mr. Schreiner the Apostle Paul would say that if Righteousness came by works of the law then Christ died in vain (Galatians 2:21) and to place one's trust in something other than Christ alone which is Faith alone is to believe in vain.