SOLVED. ancient Roman aristocrat and early Christians [s]

[Emlen]

I was reminded of this by Deanna's thread below, but I'm pretty sure it's a different book, and now I can't stop thinking about it.
It was first person, set in the late-ish Roman empire (though definitely before Constantine), and the narrator was upper-class (and maybe a decemvir or some other official?). I think it was mostly set at his villa somewhere pretty far from Rome -- I remember some mention of seeing the destruction left by recent barbarian attacks. The narrator's daughter or niece or something starts getting involved with a new religion (obviously Christianity) which the narrator finds bizarre and puzzling -- there's some mention of the fish being used as a symbol.
I thought I even knew the title: "Walking in the Garden in the Cool of the Day" (or maybe just the first or last half of that phrase), a quote from the Bible that the narrator hears and is fascinated by. But Google says no, and it's possible my memory is confusing two books. I also seem to remember reading this in translation -- maybe from Portuguese? Or Spanish?
I know I read the book some time between about 1993 and 2000, and I think it was recently-published when I read it. Help!

[Abigail]

A God Strolling in the Cool of the Evening? It is translated from the Portuguese, and the descriptions I read sound pretty close to what you put here.

[Emlen]

Yes! Thank you!

[Abigail]

You're welcome! :)