Cap. III Sol. Rat. 6: Aquinas — justice presupposes mercy; first gift free adoption
For as Aquinas puts it best (ST I q.21 a.4): every work of divine justice always presupposes a work of mercy on which it is founded. So we must not run to infinity — we have to arrive at some first thing that depends on the mercy and goodness of the divine will alone. And what else can that be than the gratuitous adoption and destination of the elect to eternal life?
Latin
Nam, ut optime Aquinas Part. I. qu. 21. art. 4, Omne opus Divinae justitiae semper praesupponit opus misericordiae, in quo fundatum sit. Nil igitur procedere velimus in infinitum, deveniamus necesse est ad aliquod primum, quod ex sola misericordia et bonitate Divinae voluntatis dependet. Hoc autem quid aliud esse potest, quam gratuita illa adoptio et destinatio electorum ad vitam aeternam?