Q:

Is it correct to use "In that" at the beginning of a sentence?

Is it correct to start a sentence using "in that"? I had a professor who would do it all the time so I started using it in papers, today someone told me it is not a complete sentence on the basis of starting the sentence with "in that". Obviously this sentence was an explanation following a statement made in the previous. 
Example: 

The emphasis by both Le Roy and Erasmus on the recovery of language
  and art as “illuminating” suggests the time before was dark and alien.
  In that both texts possess an ominous vision of the future, which
  reveals an anxiety surrounding any deviation from the dictated
  Renaissance values.

A:

In my understanding, "In that..." roughly corresponds to "given that..." or "insofar as..." 
All of these phrases could begin a sentence, but the sentence would need a clause with a main verb. Ex: "In that both texts possess an ominous vision, they are similar." In your example, there is no main verb, so it isn't a complete sentence.
I would also note: if you deleted the "in that" from your example sentence, the sentence would stand on its own, and (in my opinion) be stronger.

