John Henry Cardinal Newman 
Essay in Aid of A Grammar of Assent, An [PDF ebook] 

This classic of Christian apologetics seeks to persuade the skeptic that there are good reasons to believe in God even though it is impossible to understand the deity fully. First written over a century ago, the Grammar of Assent speaks as powerfully to us today as it did to its first readers. Because of the informal, non-technical character of Newman’s work, it still retains its immediacy as an invaluable guide to the nature of religious belief. A new introduction by Nicholas Lash reviews the background of the Grammar, highlights its principal themes, and evaluates its philosophical originality.