The Holy Spirit as the substance of love
Augustine is answering those who deny that the communion of Father and Son, called deity or love, is a substance, and he is defending the Holy Spirit's place in the one divine substance.
Some people contradict this view. They think that this communion—whether we call it deity or love or charity—is not a substance. They ask for the Holy Spirit to be explained to them according to substance, and they do not understand that it could not have been said, “God is love” (1 John 4:8), unless love were a substance. They are led astray by their familiarity with bodily things. If two bodies are joined together so that they are placed next to each other, the joining itself is not a body, since when those bodies that had been joined are separated, no joining is found. Yet it is not understood to have departed and moved away, as those bodies have. But let such people make their hearts as pure as they can, so that they may be able to see that in God’s substance there is nothing like this—as if substance were one thing there, and something else that happens to substance and is not substance. Whatever can be understood there is substance. These things can easily be said and believed, but how they actually are cannot be seen at all except by a pure heart. Therefore, whether this view is true or whether it is something else, the faith must be held unshaken: that we call God the Father, God the Son, God the Holy Spirit—not three gods, but this Trinity one God; not different natures, but of the same substance; not that the Father is sometimes the Son and sometimes the Holy Spirit, but the Father is always Father, the Son always Son, and the Holy Spirit always Holy Spirit. Let us not rashly affirm anything about invisible things as if we knew it, but as if we believed it, since they cannot be seen except by a purified heart. And if anyone sees them in this life in part, as it has been said, and in a riddle (1 Corinthians 13:12), he cannot make the person he is speaking to see them too, if that person is hindered by the filth of his heart. Blessed are the pure in heart, for they will see God (Matthew 5:8). This is the faith about God our creator and renewer.
Latin
Huic sententiae contradicunt, qui arbitrantur istam communionem, quam siue deitatem siue dilectionem caritatemue appellamus, non. esse substantiam; quaerunt autem secundum substantiam sibi exponi spiritum sanctum, nec intellegunt non aliter potuisse dici: deus dilectio est, nisi substantia esset dilectio. ducuntur quippe consuetudine rerum corporalium, quoniam, si duo sibi corpora copulentur, ita ut iuxta inuicem conlocentur, ipsa copulatio non est corpus, quandoquidem separatis illis corporibus, quae copulata fuerant, nulla inuenitur; nec tamen quasi discessisse et migrasse intellegitur, sicut illa corpora. sed hi tales cor mundum faciant, quantum possunt, ut uidere ualeant in dei substantia non esse aliquid tale, quasi aliud ibi sit substantia, aliud quod accidat substantiae, et non sit substantia; sed quidquid ibi intellegi potest, substantia est. uerum haec dici possunt facile et credi; uideri autem nisi corde puro quomodo se habeant, omnino non possunt. quapropter siue ista uera sit sententia, siue aliud aliquid sit, fides inconcussa tenenda est, ut deum dicamus patrem, deum filium, deum spiritum sanctum, neque tres deos, sed istam trinitatem unum deum, neque diuersas naturas, sed eiusdem substantiae, neque ut pater aliquando sit filius, aliquando spiritus sanctus, sed pater semper pater et filius semper filius et spiritus sanctus semper spiritus sanctus. nec temere de inuisibilibus aliquid adfirmemus tamquam scientes, sed tamquam credentes, quoniam uideri nisi mundato corde non possunt. et si quis ea uidet in hac uita ex parte, ut dictum est, atque in aenigmate, non potest efficere, ut et ille uideat, cui loquitur, si cordis sordibus inpeditur. beati autem mundi corde, quoniam ipsi deum uidebunt. haec fides est de deo conditore et renouatore nostro.