The Scallop Shell

One theory is that because Compostela is so close to the Sea, and scallop shells are plentiful in this area, early Pilgrims could easily gather the shells as souvenirs or proof of their Pilgirmage.

The 12th century Liber Sancti Jacobi relates that one of the earliest miracles of Santiago (St. James) was his rescue of a knight who had fallen into the sea and drowned. When, through the intervention of Santiago, the knight walked out of the sea, he was covered with scallop shells, and so the scallop became linked with Santiago.

The Compostela

The Certificate issued by the Archbishop of Compostela attesting the completion of the Pilgrimage to Santiago. This was important because it was the proof of the Indulgences, remission of Sins and reduction of ones time spent in Purgatory. It also could be used for various benefits, such as free lodging and meals at monastic Hospyces, etc.

Since the pilgrimage to St. James' Tomb, which spontaneously arose in the 9th
and 10th centuries it was necessary to certify its completion. For this purpose pilgrims
first made use of badges that could only be acquired in Compostela, such as the scallop
shell. Due to the number of fakes Bishops in Compostela and even the Pope had to impose
excommunication penalties against the falsifiers. From the 13th century onwards the so-called evidential letters have been used due to their well-proved efficiency. These
are the direct roots of the Compostela.

Rituals upon completion of Pilgrimage at Compostela

Indulgences

The Christians at the time believed, and so continued until very recently , that upon death one went to Purgatory and remained in this "Limbo" until sufficient time had passed to expiate one's sins. Indulgences reduced the length of this Limbo sentence.

From a 13th century document:

Relics

Relics were vitally important to medieval Christians, who believed that some of the aura of divinity associated with Christ or the saints remained in their bones, or objects associated with them. Venerating the relic, visiting it, adorning it with precious gems and metals was seen as a form of worship, and put the worshipper in direct contact with the divine. Kings preferred them to gold. Pious thieves stole them for their home churches. The power of relics drew pilgrims. Individual bones, fingers, teeth, etc., were to be found all over the Continent, were jealously guarded and even fought over. Belief in Europe's most significant relic, the nearly complete body, a unique occurrence amongst all the scattered body parts, of 1 of Jesus' 12 Apostles, launched the great pilgrimage to Compostela. Monasteries and churches all along the Camino proudly exhibited their most sacred relics to the passing pilgrims.

Miracles

The Son of Thunder, as Santiago is referred to by Jesus in the Gospels, has many miracles attributed to him:

Sometimes the intention to make the Pilgrimage to the tomb of Santiago is enough. An Englishman upon his deathbed promises to make the Pilgrimage, is immediately cured and embarks upon his Pilgrimage.

So many are the miracles of Santiago that in the Middle Ages folke made jokes and satirical songs about them. For example, a husband is walking to Santiago to petition the Apostle to let him father children. Returning home after a year he finds that his wife has a newborn child. Then, falling to his knees he prays "Cure me," Santiago.