Two lines of English are all Pope needs to put a scientist in the place of the first act of creation. But they arrive inside a Latin frame, and the frame matters as much as the couplet.
The Latin is laid out the way a monument would carry it: capitals, a name first, short indented lines. Roughly translated, it says that Isaac Newton, whom Time, Nature and Heaven declare immortal, is admitted to be mortal by this marble. The three witnesses (*tempus, natura, coelum*) are called up like testimony in a courtroom, and the verb *testantur* belongs to that world. Then comes the turn. *MORTALEM* sits alone on its own line, and the stone *fatetur*, "confesses," as if embarrassed to contradict such company. *Immortalem* and *mortalem* differ by a single prefix, so the whole paradox is one syllable wide.
The English couplet answers that confession by ignoring it. Death is never mentioned. Instead there is a before and an after, joined by a rhyme. The first line, "Nature and Nature's laws lay hid in night," moves slowly. The repeated "Nature" circles back on itself, "laws lay" drags on its soft l-sounds, and the verb "lay hid" is the most inactive thing a verb can be. The line is iambic pentameter, as heroic couplets are, and its steady beat gives a feeling of stillness, of a world waiting.
The second line breaks that stillness. "God said," opens with two heavy stresses in a row, so the rhythm lurches forward like a command. The exclamation mark cuts the line in half mid-flow, and then "and all was light" comes quickly and plainly. Look at the words: apart from "Newton," every one is a single syllable. The name is the only two-syllable word in the line, so it stands out in a stream of short, blunt ones. That is exactly where Pope wants the reader's attention.
The line also borrows from Genesis, where God says "Let there be light" and there is light. Pope changes the formula to "Let Newton be!" and the light follows as a consequence. The swap is bold, but it is carefully balanced. God is still the one speaking, so Newton is the instrument and not a rival creator. The phrase "lay hid" works in the same direction. Nature's laws were already there, waiting in the dark, and Newton uncovers them rather than inventing them. The word "laws" itself suggests, to my ear, a lawgiver behind them, which keeps the compliment within a religious picture of the universe.
"Light" does double duty. It can be the physical light that Newton studied, but it is also the light of understanding, of a mind seeing clearly. "Night" and "light," the rhyme words, are the poem's two opposites, and the rhyme chains them together so that the second line seems to cure the first. Sound itself acts out cause and effect: the couplet ends the instant darkness gets its answer.
Set beside the Latin, this gives a fuller sense of what "immortal" means here. The marble deals with the body. The verse deals with what the man did, and it is a change that cannot be undone: once the world has been lit, it cannot go back to night. Immortality becomes something Newton earned by altering how everyone after him sees, rather than a prize handed out in heaven.
The couplet is a small example of why Pope is remembered as a master of this form. The heroic couplet loves balance and neat closure, and an epitaph needs both. Here the form carries a whole attitude, a confidence in reason that Pope's age shared, expressed in the language of scripture. A modern reader might smile at "all was light," since it makes knowledge sound finished. But epitaphs are allowed to exaggerate, and the line's daring is part of its pleasure. It makes a scientific achievement feel like the moment the world began.