From Christianity and Literature, pp 1,2.
Lewis reflects on supposed "Christian" literature and art. He doesn't pontificate on the "literary value" that "written by Christians for Christians" but rather a "Christian approach to literature."
"The rules for writing a good passion play or a good devotional lyric are simply the rules for writing tragedy or lyric in general: success in sacred literature depends on the same qualitys of structure, suspense, variety, diction, and the like which secure success in secular literature.
...I question whether the badness of a really bad hymn can ordinarily be so irrelevant to devotion as the badness of a bad devotional picture. Because the hymn uses words, its badness will, to some degree, consist in confused or erroneous thought or unworthy sentiment."