When we consider the differences that exist between the Catholic and Protestant view of justification by faith, there are six key terms that represent what the Protestant Reformers were teaching. Across from these we will place six terms that represent Roman Catholicism’s understanding of justification.

For Protestants, the first key term is forensic; for Catholics it is legal fiction.

The term "forensics" has to do with speech. Protestants believe that the ultimate basis of justification involves the speech or declaration of God. Thus, when God declares a man just, he is, in fact, just. This concept is based on the scriptural passage in Romans 4 where the Apostle Paul appeals to the life of Abraham. Abraham believed God and Scripture says that God reckoned or counted Abraham’s faith as righteousness. In other words, God declared Abraham to be righteous in His sight because of his faith. For Luther and the Protestant Reformers, the basis of Abraham’s justification—and every believer’s justification—is found in God’s declaring a person fully pardoned or justified simply through faith (Romans 3:23-4:6). Thus, forensic justification is an act of God that God does outside or apart from an individual. The one who exercises faith in Christ has been officially declared righteous by God.

Roman Catholicism considers this forensic justification to be a "legal fiction." They argue it would be unjust for God to declare a sinner righteousness when, in fact, he remains a sinner. Thus, Rome teaches that God will only declare a man righteous after he works in cooperation with God’s grace, performs works of merit, and has actually become righteous.

The second key term for Protestantism is synthetic; for Roman Catholicism it is the word analytic.

By the term synthetic, Protestantism means that there is a synthesis or a combining or adding something to the life of the sinner who has placed trust in Christ. Before God’s eyes, the sinner appears clothed in the righteousness of Christ–that is, the righteousness and merits of Christ are imputed to the believer and cover him. Thus, God has declared the sinner just, not from anything in and of himself but from declaring him just in Christ. It is the merits of Christ that establish a man as righteous, not the merits of a man.

The contrary word in Catholicism is that justification is analytic; this means that God analyzes or evaluates the condition of the person, finds him to be truly righteous and therefore declares him just.

The third key word on the Protestant side is imputation; on the Catholic side it is infusion. These are a further refinement of point two.

The Reformers maintained that the merits of Christ were imputed—reckoned, accounted or transferred—from the account of Jesus Christ, so to speak, and placed into the account of the believer. In other words, the moment the sinner believes in Christ, God sees him in Christ and with all of the merits and riches of Christ applied to him. This cancels out the sinner’s debts to God. In other words, the "synthesis" has taken place. Christ and His merits have been added to the account of the believer who offers nothing of his own to God but everything of Christ’s.

It is on the basis of the merits of Christ alone being imputed to the sinner which allows God to both remain just and yet also declare the sinner just. This is why the Scripture itself declares that God displayed Christ publicly as a propitiation in His blood "for the demonstration of his righteousness at the present time, that He might be just and the justifier of the one who has faith in Jesus" (Rom. 4:26). Justification is therefore not a "legal fiction."

On the Catholic side is the word infusion. This means that the power of Christ is infused or placed into the believer. When such power is given and the believer cooperates with this power, he can arrive at a state of being just. At that time only, because the sinner really has become just, God declares him just. In other words, Rome teaches that a person must first arrive at the point where he becomes righteous within and only then will God declare him justified.

The fourth key term for Protestants is absence of human merit; for Catholics the key term is congruous merit. For Protestants, this phrase means that man has no merit of his own whatever that can dispose God to justify him. God’s justification is not based on the personal righteousness of the person or any kind of good works he can do. God’s justification is based solely upon the work of Christ.

On the other side, the Catholic phrase congruous merit teaches that when the believer works in cooperation with the grace of God, this infuses the power of Christ into a person. That person, therefore, is able to live a meritorious enough life that it makes it fitting or congruous for God to grant that person justification. In other words, man’s cooperation with Christ’s power earns man congruous merit before God.

The fifth key term for Protestants is security of justification; for Rome it would be insecurity of justification. In essence, the Reformed Protestant position is that a person, once justified, can never lose that justification. Why? Because the justification depends solely upon Christ’s merits and not anything in themselves

But this is the very reason why Rome teaches that a person’s justification is not secure. Because it does depend upon what a person does, it may, in fact, be lost by what a person fails to do. Catholicism, therefore, teaches that the commission of mortal sin causes the person’s justification to be cancelled or lost. Thus, once a person’s justification is lost, the sacrament of penance, which involves confession, absolution and satisfaction, must be undertaken in order to restore justification.

The sixth key Protestant term is "by faith alone"; for Catholicism it is "faith plus works".

Protestants maintain that justification is by faith alone, whereas Catholics say that nowhere in Scripture does the term "faith alone" occur. First, there are many things taught in Scripture that are not explicitly stated: common examples are the words "Trinity" and "monotheism" which are nowhere stated in Scripture. Justification by faith alone is derived from the fact that 1) Scripture teaches that salvation is by simple faith or trust in Christ and 2) that Scripture absolutely affirms salvation cannot be by works. Therefore, if salvation is by simple faith, and cannot be by works, the phrase, "Salvation is by grace through faith alone," cannot be considered anti-scriptural but a true presentation of what the Bible teaches. This is implied in Romans 11:6—"if it [salvation] is by grace, it is no longer by works, otherwise grace is no longer grace."

Second, for Protestants, faith is not mere intellectual assent to doctrinal truth concerning Christ’s salvation. Faith is both a knowledge of the facts and a commitment or trust or reliance upon Jesus Christ who is the sole reason and grounds upon which God justifies a person. Thus, for Protestants justification is an act that takes place in a single moment—the moment the sinner trusts Christ for salvation. At that moment the benefits of Christ are applied to his account and he is declared righteous by God. Faith is thus not a meritorious work which God considers as the reason or basis for a person’s justification; it is merely the "instrument" which allows someone to reach out to Christ Himself, the sole grounds upon which God justifies.

Catholicism teaches that justification occurs by faith and works. In defense of its view it cites James 2:24, "You see that a person is justified by what he does and not by faith alone." Protestants maintain that in context this verse cannot possibly be teaching what Catholicism says it teaches. First, because it is the only verse like it, and a hundred other verses declare justification is by faith and not by works. Protestants thus maintain that a careful evaluation of James 2 in context will reveal that it is not a denial of justification by faith alone but a complement to it. It proves that the faith that justifies, does, in fact, produce good works, although those works have nothing whatever to do with salvation. Since major portions of Romans (e.g., Romans 3:23-4:6) and Galatians (chs. 2-3) clearly teach that a man is justified by faith apart from the works of the law, the only conclusion that may be arrived at is that justification must be by faith alone. No other options exist.