We first know Magdalena as the abused wife of Buck Scales, a no-good murderer and thief. After having Scales arrested for quadruple homicide, Fathers Latour and Vaillant find Magdalena a job as a nun in a nearby convent. And once she's in the convent, Magdalena shines. She becomes so beautiful and holy-looking that Father Latour feels like he's having a religious experience just by watching her walk around a garden. For example, the sight of her prompts this vision:
At one moment the whole flock of doves caught the light in such a way that they all became invisible at once, dissolved in light and disappeared as salt dissolves in water. (7.1.33)
Father Latour is proud of many things that he's done in New Mexico, but helping Magdalena have a good life is one of the things he's proudest of. He's also happy to find that since getting away from her husband, she has grown beautiful. Or as the narrator tells us:
A handsome woman she had grown to be, with her comely figure and the deep claret colour under the golden brown of her cheeks. (7.1.33)
Magdalena is simply a good person who needed a little help to live her best life. And that's exactly what happened. Although to be fair, Father Latour and Father Vaillant actually left her behind on their first trip past her house. They didn't know her husband was a murderer at the time, but they knew enough about his abusiveness to do more for her than they did. Maybe Latour's kidding himself a bit when he thinks that he is Magdalena's savior.