θησαυρίζεις σεαυτῷ ὀργὴν ἐν ἡμέρᾳ ὀργῆς … δικαιοκρισίας τοῦ θεοῦ

Says κατά with the accusative gives the standard or cause: κατὰ τὴν σκληρότητά σου καὶ ἀμετανόητον καρδίαν, "because of your hard and unrepentant heart" (BSB). The main clause is θησαυρίζεις σεαυτῷ ὀργήν, "you are storing up wrath against yourself." σεαυτῷ is a dative of disadvantage.

Says ἐν ἡμέρᾳ ὀργῆς is "for (literally 'in') the day of wrath": the wrath is stored now and falls on that day. The day is defined by two genitives, ὀργῆς καὶ ἀποκαλύψεως δικαιοκρισίας τοῦ θεοῦ, "of wrath and of the revelation of God's righteous judgment." On the Byzantine καί before δικαιοκρισίας see var.Rom.2.5.kai-dikaiokrisias.

Abbott-Smith glosses δικαιοκρισία (dikaiokrisia) "righteous judgment," σκληρότης "hardness; metaph., of stubbornness," and ἀμετανόητος "impenitent." All three occur in the NT only here.

Implies ἀμετανόητον answers μετάνοιαν in 2:4, and θησαυρίζω reverses the "riches" (πλοῦτος) of 2:4: whoever despises God's wealth of kindness builds up a treasury of wrath. The hard heart is the opposite of the circumcised heart of 2:29.

Uncertain σκληρότης probably echoes LXX σκληροκαρδία, which Swete's LXX uses where the MT of Deut 10:16 and Jer 4:4 speaks of the heart's "foreskin" (see note.language.Rom.2.29.peritome-kardias). The shared word is the root only.

Implies The wrath of 1:18 is revealed now; this "day of wrath" is still future. The judgment then revealed is δικαιο-κρισία, a verdict that is itself righteous (compare 2:2, κατὰ ἀλήθειαν).