Some Reformed make it natural as owed to nature's order; others, like Pareus, only as concreated and not superadded
From this it is plain that many of the Reformed openly teach that original righteousness was natural to the first man because it was a perfection of the natural order, owed to a rational creature not yet corrupted by sin, shining out, as it were, from the faculties of human nature, still taken to be pure and innocent, just as health and soundness shine out from a body that is in the condition nature intends.
Others, Pareus above all, seem to call that righteousness natural to the first man only because it was a perfection created together with him and belonged to his native, original endowments. When they deny it was supernatural, they mean only that it was not something added on top of man's first, native endowments.
Latin
XXXIII. Ex his autem apparet multos ex Reformatis manifeste docere justitiam Originalem primo homini naturalem fuisse, quoniam erat perfectio ordinis naturalis, creaturae rationali peccato nondum corruptae debita, ut quae ex naturae humanae, quae pura & innocens adhuc supponitur, facultatibus veluti emicat: non secus ac sanitas & bona valetudo ex corpore secundum naturam recte se habente. Alios vero, ut Pareum in primis, videri justitiam illam eo tantum nomine primo homini naturalem dicere, quod esset perfectio quaedam primo homini concreata, & quae ad ejus nativas & Originales dotes pertinebat. Ac quum supernaturalem illam fuisse negant, non aliud sibi velle, quam illam non fuisse aliquid primigeniis & nativis dotibus hominis superadditum.