The answer to this is found in Romans 4:16:
16 For this reason it is by faith, in order that it may be in accordance with grace, so that the promise will be guaranteed to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all, - Romans 4:16
In other words, because it is a righteousness by faith it becomes sure as indicated by the word "guaranteed". Why are we saved by faith? Because then our salvation wouldn't depend upon us and our works but upon God "in accordance with grace". If we are righteous because of our works and good deeds or by following a list of rules then what happens if we fail to perform or break a rule? We become unrighteous. God would not have it! Salvation was to important of a matter to God for it to be contingent upon our performance. If it was contingent upon our performance we could do good maybe 4 days out of the week but on the 5th day blow it royally. So God has made it by grace so that it is sure, it's certain. So then it doesn't depend on my faithfulness to God but it now depends on God's faithfulness to me. And because my relationship with God depends on His faithfulness to His word, His promise, I have a sure relationship! I can come anytime, the door is never closed. When I blow it royally, when I fail miserably I can still come anytime because God isn't imputing inequity to me, God isn't taking into account my sin (Rom. 4:8). Our salvation is a guaranteed thing because it rests entirely on Him and His accomplishments (John 19:30).