"Some Romanists will say that they too teach justification by grace — by Christ’s righteousness, in fact. But the righteousness of Christ which they claim justifies is not Christ’s own personal righteousness reckoned or credited or given or imputed to believers. Romanists refer to the righteousness which Christ works into the life of the believer or infuses into him in his own living and behavior. It is not Christ’s personal righteousness but the believer’s personal righteousness, which he performs by the grace of God.
It is Christ’s righteousness versus the believer’s own righteousness. It is Christ’s achievement versus the Christian’s achievement. It is an imputed righteousness not an infused righteousness. It is a gift of God versus an accomplishment of man. These two righteousnesses are as different as righteousnesses could conceivably be.
It does come down to the way it has been popularly stated for the last four and a half centuries: Protestantism’s salvation by faith versus Rome’s salvation by works. That is not a technically accurate way to state this vital difference, But it points to the truth. The Protestant trusts Christ to save him and the Catholic trusts Christ to help him save himself. It is faith versus works. Or, as the Spirit of God puts it in Romans 4:16 (NIV), "Therefore, the promise comes by faith, so that it may be by grace, and may be guaranteed to all Abraham’s offspring." It is "by faith SO THAT IT MAY BE BY grace...."
If a Romanist wants to be saved by grace alone, it will have to be by faith alone. "The promise comes by faith so that it may be by grace." You can’t be saved "sola gratia" except "sola fide"...
...We agree with Roman friends — salvation is by grace. That is the reason it must be by faith. If it is a salvation based on works that come from grace, it is not based on grace but on the Christian’s works that come from grace. The works that come from grace must prove grace but they cannot be grace. They may come from, be derivative of, a consequence of, but they cannot be identified with it. Faith is merely union with Christ who is our righteousness, our grace, our salvation."
-Excerpt from Justification by Faith Alone by Dr. John H. Gerstner