The façade is divided into two distinct sections, the upper one consisting of a Baroque pediment added in the 18th century and topped by a sculpture of Saint Isidore on horseback, alluding to the tradition of the banner of Baeza, which is preserved in the museum. The Romanesque façade itself, known as the Puerta del Cordero, features two semicircular archivolts decorated with fluted moldings on columns, while a third segmented archivolts rests directly on the jambs and is framed by a checkerboard-patterned molding. Ornamental moldings with scrolls and palmettes are interspersed among the archivolts, and the columns, arranged in a crouching posture, rest on Attic bases atop very high plinths that do not correspond to the originals.

The capitals display rich and expressive sculptural decoration, featuring human figures with claws instead of hands and feet, winged figures, crouching figures, and plant motifs. The tympanum, considered the oldest in the Kingdom of León and dating to the High Romanesque period of the 11th century, is carved from white marble and rests on jambs topped with ram’s heads. It depicts the Sacrifice of Isaac, presided over by the mystical lamb held by two angels, while two others carry the symbols of the Passion of Christ, following an iconography widespread in Mozarabic Hispania, where this scene frequently replaced the Crucifixion.

On the right are Sarah at the entrance to the tent and Abraham’s two servants, one on horseback and the other removing his shoes as a sign of respect before a sacred place; Isaac is also depicted barefoot, with his sandals visible on the ground. Abraham listens to the divine voice, symbolized by the Dextera Domini, while the sacrificial ram appears caught in the undergrowth, accompanied by an angel. On the left side are the figures of Ishmael and his mother Hagar, with Ishmael depicted as an archer—an exceptional representation found almost exclusively in 11th-century Hispania. The entire relief is carved from local stone, although marble sculptural pieces were reused in the spandrels of the arch, likely originating from the church of Fernando and Sancha; their current arrangement is inconsistent, indicating that they were relocated during a later restoration.

Thus, the statue of the martyr Pelayo holds a book that would correspond to Saint Isidore, while next to the latter a executioner with a knife was added, which in reality alludes to Pelayo’s martyrdom; Saint Isidore is identified, however, by the inscription with his name next to his head. The Museum of San Marcos houses a related sculpture depicting Saint Vincent of Avila, whose relics also arrived at the church in 1063. Above these figures are reliefs showing King David accompanied by five musicians and a violinist framed by concentric circles, while above Pelayo are a violinist and a drummer.

Above this runs a frieze interrupted by the outer scroll of the archivolt, depicting the signs of the zodiac arranged from right to left. These were originally identified by inscriptions from the second half of the 11th century, some remnants of which are preserved outside their original location, once again confirming the reuse of sculptural elements from the previous building.