1. Because we are justified by the object of faith alone, that is by the merits of Christ only, without which we can have no righteousness whatever: for we are justified for Christ’s sake. Nothing but the merit of Christ can be our righteousness in the sight of God, either as a whole, or a part only. We are justified only by believing, and receiving the righteousness of another, and not by our own works, or merit. All works are excluded from, our justification, yea even faith itself in as far as it is a virtue, or work.
2. Because the act which belongs properly to faith is to apprehend, and apply to itself the righteousness of Christ; indeed, faith is nothing else than the acceptance itself, or the apprehension of the merits of Christ.
3. Because faith alone is the instrument which apprehends the satisfaction of Christ. Hence it is plain, why the exclusive particle only should be added, as it is in the Catechism, and be maintained against the Papist. It is done,
3.1. For the purpose of expressing what Paul affirms when he says: “We are justified freely by his grace, without the deeds of the laws:” And what Christ says; “only believe.” (Rom. 4:24, 28. Mark. 5:36.)
3.2. That all our own works, and merits, as well as those of others, may be excluded as being the cause of our justification, that faith may be understood correlatively. We are justified by faith only, that is, by the merits of Christ alone.
3.3. That not only all our merits, but that even faith itself may be excluded from that which is received by faith; so that when we say, we are justified by faith only, the sense is, that it is not by meriting, but only by receiving; as when it is said, This beggar is enriched only by receiving alms, all works and merits are excluded there from, yea, even the very acceptance of alms, in as far as it is viewed as a merit. It is for this reason, that Paul always says, that we are justified by faith, and through faith, as by an instrument; and never on account of faith, as the Papists will have it, who indeed admit both forms of expression, as if faith might be the application of Christ’s righteousness, and be also at the same time a certain work, or merit, by which we are counted worthy of being declared righteous, which is directly opposed to the very nature of faith. For if we were justified on account of our faith, then faith would no longer be the acceptance of the righteousness of another, but it would be the merit, and cause of our own righteousness; neither would it receive the satisfaction of another, for it would no longer stand in need of it.
3.4. That we may understand the necessity of faith for our justification, and may know that we are justified, not by the merit of faith, but yet just as little without faith, to receive the righteousness of Christ; because it is the province of faith to appropriate this to itself.
3.5. The orthodox Fathers often use the same form of speech, by faith only. Origen writes: “The Apostles say, that the justification OF FAITH ONLY is sufficient, so that if any one ONLY BELIEVES, he may be justified, even though he does not perform any works.” Ambrose says: “They are justified freely, who, with out working or rendering any thing in turn, are justified BY FAITH ONLY as the gift of God.” Again; “How can the Jews suppose that they are justified by the works of the law, seeing they have the justification of Abraham set before them, who was justified, not by the works of the law, but BY FAITH ONLY. The law, therefore, is not necessary, when the sinner is justified before God by FAITH ONLY.” And again. “God has decreed that he who believes in Christ, should be saved without works, receiving the remission of sins freely BY FAITH ONLY.” We are therefore justified by faith only, which means that it is by the merits of Christ alone, apprehended by faith.
This we must firmly maintain, and believe:
3.5.1. For the glory of God, that so the sacrifice of Christ may not be impaired.
3.5.2. For our consolation, that we may be assured that our righteousness does not depend upon our works, (for if this were the case we should lose it thousands of times,) but upon the sacrifice and merit of Christ alone.
—Zacharias Ursinus, Compendium of Orthodox Doctrine (on Heidelberg Catechism 61).