Updated: May 18, 2021
12. This Epistle is undeservedly accused of obscurity by some of the ancients, Pererius, and others.
Some of the ancients accuse this Epistle of obscurity. Origen, in Philocalia, chapter 9, says: προκεῖσθαι κρύψαι, καὶ μὴ φανερῶς ἐκθάσθαι τὰ νοήματα τῆς ἀληθείας τῷ ἐν τοῖς προφήταις πνεύκατι, καὶ ἐν τοῖς Ἀποστόλοις Χριστοῦ λόγῳ, it was proposed to the Spirit, who was in the Prophets, and to the word that was in the Apostles of Christ, to hide, and not manifestly to express the sense of the truth; and he adds that this was especially done in this Epistle to the Romans, since it has more difficulty than the others. And Pererius does not fear to accuse the speech of this Epistle as entangled and confused, its expressions as uncertain and ambiguous. But there is no danger that it might not be understood by those that present themselves as teachable to the Holy Spirit, who give due προσοχῇ/attention, Hebrews 2:1, and who, drawing out τὰ στοιχεῖα τῆς ἀρχῆς τῶν λογίων τοῦ Θεοῦ, the first principles of the oracles of God, are not ἄπειροι λόγου δικαιοσύνης, unskillful in the word of righteousness, Hebrews 5:12-6:1; finally, who resolve to explain all things according to the analogy of faith, and to hold fast wholesome words. Accordingly, those that complain of the style of this and the other Epistles, and exaggerate the difficulties (which even Grotius very studiously does), render themselves suspect of supine negligence, and that they are prepared to obtrude their own, that is, better, words in the place of the words of the Holy Spirit.
 Hebrews: 2:1: “Therefore we ought to give the more earnest heed (δεῖ περισσοτέρως ἡμᾶς προσέχειν) to the things which we have heard, lest at any time we should let them slip.”  See 2 Timothy 1:13.
Dr. Dilday's Lecture: "Romans, Part 4"