Cap. 7 Contra Sol. 1: mercy fulfilled through Christ's merits
These two claims do not clash: God justifies and saves Peter by pure grace — his mere eudokia — and God justifies and saves Peter with an eye to Christ's merits specially ordained and accepted for Peter's salvation. The reason is plain. The same God who, from bare good pleasure and pure mercy, decreed faith and salvation for Peter also decreed to give them only through and because of Christ's merits, destined and granted to Peter in a singular way — and that from the same eudokia and pure mercy. God's good pleasure stands against our merits; yet through Christ's merits applied to us it is filled out, not voided. So the Apostle says (Hebrews 13:21) that God himself works his good pleasure in us through Jesus Christ. It is therefore mere Divine eudokia that God wills to give Peter efficacious grace and save him; still that good pleasure is carried out only through Christ's merits. Our whole answer comes to this: every work of special grace in the elect is for Christ's merits specially applied to them, yet their justification and salvation show true mercy, not justice — because Christ, with his merits and with this very way of meriting and offering them under a special regard for the elect, drew the whole plan from God's free will toward them.
Latin
Nulla pugna est inter haec duo pronunciata, Deus justificat & salvat Petrum ex pura gratia sive mera sua eudokia, & Deus justificat & salvat Petrum intuens merita Christi pro Petri salute specialiter ordinata & acceptata. Ratio hujus rei in promptu est; nempe, Quod ipse Deus, qui ex mero suo beneplacito & pura misericordia decrevit dare Petro fidem & salutem, decrevit simul non aliter illi haec dare quam per & propter merita Christi Petro singulari quadam ratione destinata & donata, idque ex eadem eudokia & pura misericordia. Quae Dei eudokia opponitur meritis nostris; sed per merita Christi nobis applicata impletur, non evacuatur. Unde Apostolus Heb. xiii. 21. dicit ipsum Deum efficere in nobis beneplacitum suum per Jesum Christum. Quamvis igitur sit mera Dei eudokia, quod Deus velit Petrum efficaci gratia donare & salvare, tamen hujus beneplaciti executio non aliter fit quam per merita Christi. Huc igitur redit summa responsionis nostrae; Quamvis omne opus specialis gratiae quod fit in electis, fiat propter merita Christi specialiter illis applicata, tamen in illorum justificatione & salute veram rationem misericordiae, non justitiae conspici, quia Christus cum meritis suis, cum hoc ipso modo merendi, & offerendi sua merita speciali quodam intuitu habito ad electos, totum, inquam, hoc ex gratuita Dei erga illos voluntate duxit originem.