The Chrismon was an early Christian symbol which generally consisted of an X superimposed over a cross bearing the Greek letters Alpha and Omega on each arm.
While it largely disappeared from Western European Christian art of the medieval period, the Chrismon continued to be used in Spain and its near ubiquitous presence in Romanesque Aragonese sculpture arises from a specific set of circumstances.
The Chrismon was first and foremost an adaptation of a Roman military standard called the Labarum. This was a standard topped with a draped banner surmounted by a Chrismon. In Christian legend it would be associated with the account by the historian Eusebius of the Roman Emperor Constantine’s victory at the battle of the Milvian bridge. Later, at the battle of Adrianople Constantine ordered it to be deployed to whichever part of the field his troops were struggling, such was its talismanic value.
A second Constantinian connection with the Chrismon arose from its function as a symbol of the Trinity, Constantine having presided over the formulation of the Trinitarian Creed at the Council of Nicaea in 325.
In Spain, however, the Visigoths maintained the Arian heresy, denying the Trinity long after it was outlawed in the rest of Christendom. It took a further two hundred and sixty years before the Nicene creed was adopted at the Council of Toledo in 589. Presided over by King Reccared, a contemporary chronicler declared that at the council the king, “Like Constantine among the bishops of Nicaea, anathemitsed Arius”.
This must goes some way in explaining why the Chrismon appears to have held particular meaning for late period Visigoths before the Arab invasion, appearing in numerous instances all over the peninsula.
The use of the Chrismon began to enjoy a ressurgence once more in Spain in the ninth century coinciding with the first period of the Reconquista. A Chrismon is to be found in the apse of the church of San Salvador de Valdedios built by Alfonso III in 893.
This followed a tradition which began with the first Christian victory against the Saracens at Covadonga in 722, when it was said that the leader of the small Christian force, Pelayo went into to battle under the banner of the Chrismon, in clear imitation of Constantine.
Whether this is true or not, the association was perpetuated by his heirs who forged the kingdom of Asturias in the wake of Pelayo’s victory. In 908 a gem encrusted cross said to be the original wooden cross which Pelayo had carried to victory was donated to the cathedral of Oviedo by his descendant King Alfonso III. Crucially this cross bore the inscription “By this sign you shall conquer your enemies”, which intentionally echoed Constantine’s motto for the Chrismon. Alfonso’s Victory Cross also bore the Alpha and Omega of the Apocalypse at each end of the horizontal arms, as did the Romanesque Chrismons of northern Spain.
Thus, the Spanish kings of the Reconquest saw themselves as the natural heirs of Constantine, victorious Christian rulers and defenders of the true faith. By the eleventh century the Chrismon was prevalent in the sculptural decor of Navarrese and Aragonese churches, the two kingdoms most associated with the early Reconquista and were particularly associated with royal establishments.
Sources: Canellas-Lopez A.- San Vicente A. · Aragon Roman Edition Zodiaque 1971. Collection la nuit des temps 35
R. Bartal, The survival of early Christian symbols in 12th century Spain. Principe de Viana 48 (1987), S. 299-315