Faith is never the basis or the reason for justification, but only the channel through which God works His redeeming grace. Faith is simply a convicted heart reaching out to receive God's free and unmerited gift of salvation.
The story is told of a poor farmer who had saved his money for years in order to buy an ox to pull his plow. When he thought he had enough saved, he traveled a great distance to the nearest town to shop for an ox. He soon discovered, however, that the paper money he had been saving had been replaced by a new currency and that the date for exchange from the old to the new had long since passed. Because he was illiterate, the man asked a neighbor school boy to write a letter to the president of their country, explaining his dire situation and asking for an exemption. The president was touched by the letter and wrote back to the farmer: "The law must be followed, because the deadline for exchanging bills has already passed. The government can no longer change your bills for the new ones. Even the president is not exempt from this rule. However," the president continued, "because I believe that you really worked hard to save this money, I am changing your money for new money from my own personal funds so that you will be able to buy your ox."
Before God, every person's good works are as worthless as that farmer's outdated money. But God Himself, in the Person of His Son, has paid the debt. And when a confessed sinner casts himself on God's mercy and accepts in faith the Lord's atoning work in his behalf, he can stand forgiven and divinely righteous before Him.
(from The MacArthur New Testament Commentary, (c) Moody Press and John MacArthur, Jr., 1983-2002)