I've been an admirer of the poetry of Edna St. Vincent Millay for a while. She is a modern master of the sonnet. Consider this:
What lips my lips have kissed, and where, and why,
I have forgotten, and what arms have lain
Under my head till morning; but the rain
Is full of ghosts tonight, that tap and sigh
Upon the glass and listen for reply . . .
Or her poem "Grown-up":
Was it for this I uttered prayers,
And sobbed and cursed and kicked the stairs,
That now, domestic as a plate,
I should retire at half-past eight?
I did not know what kind of person she was, and I always assumed she was something like Sylvia Plath. So I was bit surprised to see her described by Dallas Willard as a "Christian poet." Intrigued by this possibility, I read her biography by Nancy Milford. It turns out that unless the seven deadly sins have been repealed, Millay's standing among the saints is nonexistent. Of course, when one says this, it must always be with the caveat that only God knows the heart. Perhaps her behavior belied her Christian beliefs, but if Milford's biography is comprehensive (which I think it is), then Millay never seems to have had much interest in Christianity.
Her life was full of impropriety, but no life is untouched by that, so hers is not distinguished in that respect. But contrast her life with that of another public figure, Johnny Cash, and one sees that the unpleasantness and impropriety of her life seemed to mount at the end of her life, as if her life had been propped up by youth and vigor then fell into disarray at the end. On the other hand, Cash's youthful antics are well-known, but he eventually renounced them for a life of virtue, and his later work seems to have been just as good (if not better) than his earlier. This is due, I think, to the fact that he corralled his dissipation in middle age. Millay did not, and her work seems to have suffered from this, though I am no expert on her work. Cash's happiness at the end of his life is plain; Millay's life did not end very happily. (There may also be something to Cash's being raised by devout Christian parents whereas Millay was not. Cash's wife, June Carter, also strikes me as more virtuous, perhaps because more stubborn, than Millay's husband, Eugen Boissevain.)
Now I think it is possible for Millay to have written Christian poetry without having been a Christian herself. Her poetry does contain Christian symbolism and themes, and the poem Willard cites ("Feast") seems to be evidence of this. So she may be a Christian poet in that respect. I would prefer to say that her poetry is Christian while she herself may not have been. (Note that her middle name, "St. Vincent," is a reference to the hospital where she was born. Many of her close friends called her "Vincent" or "Vince.")
Milford's biography, by the way, was a good read, but she seemed to commit what C. S. Lewis called the "personal heresy" quite a bit: the clue to understanding Millay's poetry is to understand part of her biography. I disagree with this principle, but that's for another day.