From the LCMS website:
Constantine I served as Roman Emperor from A.D. 306 to 337. During his reign the persecution of Christians was forbidden by the Edict of Milan in 312, and ultimately the faith gained full imperial support. Constantine took an active interest in the life and teachings of the church and called the Council of Nicaea in 325 at which orthodox Christianity was defined and defended. His mother, Helena (ca. 255-329), strongly influenced Constantine. Her great interest in locating the holy sites of the Christian faith led her to become one of the first Christian pilgrims to the Holy Land. Her research led to the identification of Biblical locations in Jerusalem, Bethlehem, and beyond, which are still maintained as places of worship today.