The formula it is . . . that is one of the most common rhetorical tics in academic writing. This formula also provides a great opportunity to edit for concision, since it can usually be removed easily from a sentence without changing the meaning.
Often, one can simply remove “It is” and “that” and leave the sentence otherwise untouched.
It is this ambivalence that gives the poem its power.
This ambivalence gives the poem its power.
Sometimes, it may work better to place the phrase that follows “It is” at the end of the sentence.
It is in the light of this discovery that we should reexamine our understanding of T. S. Eliot’s early poems.
Edited version A:
We should reexamine our understanding of T. S. Eliot’s early poems in the light of this discovery.
Some sentences, like this one, lend themselves to a more radical overhaul. Here is one way to make this sentence even more concise and dynamic.
Edited version B:
This discovery compels us to look at T. S. Eliot’s early poems in a new light.
This formula, however, is not intrinsically bad. Used sparingly, “it is . . . that” can be an effective rhetorical tool. A writer can use it to pluck an otherwise unremarkable phrase from the surrounding prose and unveil it before the reader with a flourish. Jane Austen comes to mind:
It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.
Few readers have ever wished that Austen had begun Pride and Prejudice this way instead:
We all know that a single man with a lot of money must need a wife.