The theme of Have A Little Faith by Mitch Albom is how faith and love can make us better people. Have A Little Faith is a nonfiction book, focused on the author's encounters with two spiritual leaders, one Jewish (Albom's own faith tradition) and one Christian.
Rabbi Albert Lewis is dying and asks if Albom will give his eulogy. As Albom talks with Lewis he discovers not only the background of Lewis' life but also embarks on a journey of self-discovery. The second major character in the book is Henry Covington, a former drug dealer who has converted to Christianity and is in the process of running a ministry for the homeless and destitute in Detroit. As he talks with both these men, Albom becomes increasingly convinced of the importance of faith in people's lives.
As for why Albom addresses these themes, one can assume that since his similarly inspirational earlier book, Tuesdays with Morrie, was a major bestseller, that he assumed (correctly) that a similarly inspirational book would be equally lucrative.