“The righteousness with which we are here justified before God, is not our conformity with the law, not our good works, nor our faith; but it is the satisfaction which Christ rendered to the law in our stead; or the punishment which he endured on our behalf; and therefore the entire humiliation of Christ…whatever he did…is all included in the satisfaction which he made for us, and in the righteousness which God graciously imputes to us, and all believers” (Zacharius Ursinus, Commentary on the Heidelberg Catechism trans. G. W. Williard [Phillipsburg: P&R, n.d.], 327).
“How can man, being a sinner, be just before God?…Man as a sinner can be regarded as righteous only on the ground of the imputation of Christ’s merits; and this is the question of which we speak when treating the subject of justification” (ibid, 328).
This is crystal clear; there are no ambiguities for Ursinus when it comes to justification, righteousness, and imputation. None.
For more examples, see pages 110, 289, 305, 322, and 495 of Ursinus’ commentary.