[2: 72, verses 6-8]
. . .
O Mar dove ogni mente indarno spalma,
O Sol dove ha suoi lumi invano affissi,
O tenebre lucenti, o sacri abissi!
O Sea where each mind sails in vain,
O Sun upon which it gazes uselessly,
O shining shadows, O sacred abysses!
The so-called "negative" or "apophatic" theology, i.e. unable to describe anything of God, has a long tradition in the history of Christianity: from the very beginning to nowadays, as a matter of fact, across the very different developments in society and thought. And it gains a special strength in Tasso's Baroque worldview, with a more existentially tragic nuance than, e.g., in Dante.
For this kind of heavenly and at the same time hellish atmospheres, see, still in the early 19th century, Giuseppe Gioachino Belli's sonnet La ggirànnola der 34: "lusce nera . . . fiamme illuminate . . . e ll'Angelo . . . pareva un demonio de l'inferno," black light, illuminated flames, the (statue of the) angel looking like a devil during a firework show. Unconsciously echoing Milton, or maybe consciously?
As to language, Tasso once again uses two synonyms of the same word in two verses, "in vain" in this case, with a chronological difference: one of them is the (now) outdated form "indarno," while the other, "invano," is the form still commonly used.