Did Christianity borrow from Mithraism?
The short answer
It seems the reverse. Mithraism had some similarities to Christianity, but our knowledge of it is very imperfect, and in a number of details Mithraism was probably the borrower from Christianity.
Similarities
Mithraism appears to have had many similarities to Christianity.
Who borrowed?
The Catholic Encyclopedia notes that our knowledge of Mithraism is very imperfect, mostly guesswork, and that we know nothing of how its adherents understood it when Christianity arose. Some similarities exist, but in a number of details it is quite probable that Mithraism borrowed from Christianity.