Which parts of the bible might resonate with a little kid? Psalms? Gospel of John? The four evangelists? Hymns?
I’m a Celtic Pagan born and bred myself, but when my widowed dad married a Christian when my brother and I were in our early teens I made a serious study of Christianity so that I could have a civilised discussion with her about why we were not going to automatically convert to her faith. I read both Testaments thoroughly, several times, managed to track down parts of the apocrypha, read all the Christian theology I could lay my hands on, and discussed matters at length with friends who were a devout Catholic, an Anglican priest, an elderly lady who wrote hymns and had been a missionary, the son of a Mormon bishop, and one who was a non-practising Baptist. I learned a lot. I recommend CS Lewis (yes, as in the Narnia books) as an interesting writer on modern Christian theology. Both ‘The Problem of Pain’ and the Screwtape Letters are worth a read.
Maybe your character knows hymns sung to her by her parents? Like the little kid’s ‘Now I Lay Me Down To Sleep’? Or maybe her guardian angel may decide that the most useful thing it can do for her is facilitate her training as a mage? I see no reason why magic and Christian faith need be mutually exclusive, having known several Christians and Christian priests who actually practised magic in our time and world (mostly of the healing kind). And remember Pastor A, who was probably a Lutheran pastor?