'a key passage' is correct and usable in written English.
You can use the phrase to describe a section of a text which is particularly important or informative. For example, "The key passage in this novel occurs when the protagonist makes an important decision."
In a key passage, he writes: "No, the jungle wasn't evil.
"I used to imagine what it would be like to kill someone," he admits in a key passage.
In a key passage, he writes: "Now I know — you know, we all know — that the fans were right.
A key passage of the encyclical asserts, "Each and every marriage act must remain open to the transmission of life".
At his most helpful, he sets up themes and procedures of the narrative, then brings them to bear in the reading of a key passage.
In a key passage, the leading article stated: "To all these embittered Remainers, the Mail has a simple message: You lost.
The Lish version, retitled "The Bath," is a third shorter and eliminates the original story's key passage, a moment of redemptive hopefulness at the end; instead, the couple are left frozen in despair.
With a new trial set in Florence in the coming year, a few key passages of "Waiting to be Heard" will most likely be heard once again, in an Italian courtroom.
Another is that period brasses, which sound more bronze than the bright golden shine of modern instruments, are harder to control, and a few key passages in the performance of Mendelssohn's complete incidental music for "A Midsummer Night's Dream" suffered as a result.
Mentioned by Homer in a few key passages, Achilles's male lover Patroclus is the novel's narrator and its heart.
With hindsight, however, it is possible to see that a few key passages within these instructions would not only fail to do anything to supress the use of torture but might even facilitate torture and encourage it.
