I have not spoken in secret, in a dark place of the earth: I said not unto the descendants of Jacob, Seek me in vain: I the LORD speak righteousness, I declare things that are right.
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Cyril of Alexandria
“I speak righteousness and proclaim truth.” Righteousness, as I take it, means the law, for the law is the arbitrator of righteousness. And truth is the discipline of Christ. For there is a form of truth in the law, and the mystery of Christ lies concealed in its shadows, for Moses wrote about this. So, in speaking of righteousness, truth and power are proclaimed along with it, since it is true that the law is a shadow—and yet in it there remains a form of the truth. - "Commentary on Isaiah 18.104.22.168–19"
I drew up laws about these things for them and did not speak in riddles, nor did I speak in a weighty tone but lightly and clearly. As a result, the Lord came down on Mount Sinai in the form of fire. There were darkness, storms, smoke and the piercing sound of trumpets. - "Commentary on Isaiah 22.214.171.124–19"
Earth. The pagan oracles were given chiefly in mountains, where the impostures of the priests might escape detection. They were also generally ambiguous, or mere guesses. The declarations of the true prophets were quite the reverse.
In vain; without reward. (Calmet)