"How much faith do you need in order to be saved and how do you measure it? Can you read anyone's heart? Can you even read your own?"
True faith need only be the size of a mustard seed, and its growth is assured by God (Matthew 13:31). A genuinely saving faith will be demonstrated through good works. Those who claim to trust in Christ as Lord and Savior yet reject core doctrines, show no repentance of sin, or display no desire to grow in sanctification have every reason to question their standing with the Lord.
The author claims that proponents of justification by faith alone (Sola Fide) are guilty of making two assumptions, which will be examined here as follows:
"That they have the right to declare themselves saved."
God promises to save the people who believe on Him for salvation. Faith involves trusting what He says. If God tells us that all who believe are saved, then it is not a false assumption that one is saved because the person has faith. To deny this is to call God a liar.
"That they have the ability to measure their faith."
Measuring faith is not by any means a requirement. It is not the amount of faith that saves. A weak faith can latch on to a strong Savior. It is not faith that saves us but a Person who saves through faith.
Recognizing our status with God is not tantamount to declaring ourselves righteous in His sight.
"The proof is very easy to discover. Ask a Catholic, any Catholic, whether he is saved. His answer will invariably be, "I don't know." Why? Because God is our Judge."
This is the very heart of the problem. How could we possibly live a life of joy and peace, if we are not able to have assurance of eternal life? The god of Islam gives adherents no assurance of salvation, which is no doubt a cause of much fear and uncertainty.
The very point of asking people how many works are necessary for salvation is to get them to reconsider their standing with the God of the Bible.
The New Testament clearly indicates that Christians can have assurance of salvation. Consider the text of John 10:27-29. 1 John 1:7-9 assures that repentant people are forgiven (in the present tense) by God. Consider also the text of 1 John 5:13-15.