Witsius cites Calvin’s Institutes 3.17.8 in support of the tradition of double justification. In section 8, Calvin, after defending justification by faith alone, does teach a justification by works which is itself founded on the prior justification by faith alone. Calvin writes:
Forgiveness of sins being previously given, the good works which follow have a value different from their merit, because whatever is imperfect in them is covered by the perfection of Christ, and all their blemishes and pollutions are wiped away by his purity, so as never to come under the cognizance of the divine tribunal. The guilt of all transgressions, by which men are prevented from offering God an acceptable service, being thus effaced, and the imperfection which is wont to sully even good works being buried, the good works which are done by believers are deemed righteous, or; which is the same thing, are imputed for righteousness.
Notice that Calvin rejects that these works, which are imputed for righteousness, have any merit. They are acceptable, even counted righteous, but they are not meritorious. John Davenant will follow him in this very position in his treatise on inherent and imputed righteousness.
Calvin continues in sections 9 and 10 of the same chapter on this same topic. He states:
They cannot deny that justification by faith is the beginning, the foundation, the cause, the subject, the substance, of works of righteousness, and yet they conclude that justification is not by faith, because good works are counted for righteousness. Let us have done then with this frivolity, and confess the fact as it stands; if any righteousness which works are supposed to possess depends on justification by faith, this doctrine is not only not impaired, but on the contrary confirmed, its power being thereby more brightly displayed. Nor let us suppose, that after free justification works are commended, as if they afterwards succeeded to the office of justifying, or shared the office with faith. For did not justification by faith always remain entire, the impurity of works would be disclosed. There is nothing absurd in the doctrine, that though man is justified by faith, he is himself not only not righteous, but the righteousness attributed to his works is beyond their own deserts.
In this way we can admit not only that there is a partial righteousness in works (as our adversaries maintain), but that they are approved by God as if they were absolutely perfect. If we remember on what foundation this is rested, every difficulty will be solved. The first time when a work begins to be acceptable is when it is received with pardon. And whence pardon, but just because God looks upon us and all that belongs to us as in Christ? Therefore, as we ourselves when ingrafted into Christ appear righteous before God, because our iniquities are covered with his innocence; so our works are, and are deemed righteous, because every thing otherwise defective in them being buried by the purity of Christ is not imputed. Thus we may justly say, that not only ourselves, but our works also, are justified by faith alone. Now, if that righteousness of works, whatever it be, depends on faith and free justification, and is produced by it, it ought to be included under it and, so to speak, made subordinate to it, as the effect to its cause; so far is it from being entitled to be set up to impair or destroy the doctrine of justification.
So we can now add Calvin to our list of Reformed doctors who taught some form of double justification. Witsius also cites Bucer, whose Loci Communes I have ordered (it is checked out from the library and says it won’t be back for a year!). Bucer’s double justification is clear though, and most people know about it.
The other important thing about the selections from Calvin is that they were noticed in their day and cited by later Reformed doctors. This was a legitimate part of the tradition.