Located in the Helecha neighbourhood, in La Serna (Arenas de Iguña), it stands as one of the oldest Romanesque temples in Cantabria.
Since the 12th century it belonged to the Order of the Knights of Saint John of Jerusalem, whose function in this area was to control the route of the Besaya valley, which linked Castile with the Cantabrian coast and which at that time was very busy with pilgrims and merchants.
Since its construction, the temple has undergone numerous reforms: in the 15th century the chapel was added and the nave was extended towards the foot, removing the Romanesque walls.
The access door, with a pointed arch and simple decoration, is located on the west wall. Above it is an oculus-shaped window and, as a finishing touch, a bell gable with a double pointed arch loophole. The remains from the Romanesque period are limited to the main apse, with a semicircular floor plan, on the eaves of the outer wall of which there is a row of undecorated lobed corbels, in the Mozarabic tradition. This apse is covered with a barrel vault, preceded by a semicircular triumphal ship with simple capitals. On the capital on the Gospel side there are a kind of engraved volutes that could be some plant form. The tops of both capitals are decorated with half palmettes.