FORT WORTH — Monica Lopez knows the power of devotion to Our Lady of Guadalupe. Four years ago, the mother of three school-age children was diagnosed with stage four breast cancer. She believes praying for the intercession of the Virgin Mary, who appeared to Juan Diego in 1531, spared her life. “That’s why I’m here,” said Lopez, a parishioner at Assumption of the Blessed Virgin Mary Parish in Decatur. “I know she’s been at my bedside. I could picture her, see her, feel her. She helps us.