I read all the Narnia books as a child, but they were never specifically written or marketed as Christian fiction, so I don’t think they count.
I also read and enjoyed Joni by Joni Eareckson in my early teens (so only a few years after it was first published in 1976). That’s definitely Christian, but is nonfiction so definitely doesn’t count.
The first overtly Christian novel I remember reading was This Present Darkness by Frank Peretti, which I read when I was about eighteen. My aunt bought it and loaned it to my mother (who never read it). I read it and the sequel, Piercing the Darkness, and the duology got me started on many happy years reading Christian fiction.
What about you? What’s the first Christian novel you remember reading?