The lion, the bull, the eagle and the angel: the Tetramorph

The word ‘tetramorph’ etymologically indicates a representation of four elements.  In Christian tradition, the prophet Ezekiel describes four creatures with a human face and animal appearance.  Already in the Middle Ages, the four evangelists were associated, represented around Christ.  In the Romanesque art rooms, there are various notable examples, both in mural painting (the one of Sant Climent de Taüll), as well as in panel painting, such as the one of Tost Baldachin.

The lion represents Mark because his Evangelist starts speaking of Saint John the Baptist, who preaches in the wilderness.  His voice is like that of the lion, a strong and noble animal, as Jesus would be.   

The bull is Luke, because he starts talking of the sacrifice of Zachariah to God and the bull is the symbol of sacrifice, the desire for a spiritual life, which allows man to triumph beyond animal passions and to obtain peace. 
The eagle symbolises John because it is an animal which is considered to be wise and clairvoyant, which when it flies it looks straight at the sun, and the evangelist of John is more abstract and theological than the others.
Finally, the angel is Matthew, because he is the only one who talks of the genealogy of Christ, the Son of the Father, and moreover represents divine love, sent by the angles (the messengers of Christ) to the humans.