At the time, many Copts (Coptic Christians) in Egypt were engaged in handicrafts. Saint Simon worked in one of those crafts widespread in Babylon (Old Cairo) which was tanning, a craft still known there till this day. This profession involved also other crafts that depend on the process, from where St. Simon carried several titles related to skins; Saint Simon the Tanner, the Cobbler, the Shoemaker.
The Caliph Al-Muizz used to invite different religious leaders to debate in his presence. In one of those meetings in which Pope Abraam and a Jew named Yaqub (Jacob) Ibn Yusuf Ibn Killis were present, the Pope got the upper hand in the debate. Plotting to take revenge, Ibn Killis quoted the verse where the Lord, Jesus Christ, said in Matthew: If you have faith as small as a mustard seed, you can say to the mountain, Move from here to there, and it will move, nothing will be impossible for you and demanded that the Pope prove that his religion is right by means of this. The Caliph saw in this a unique opportunity and asked Pope Abraam to move the Mokattam Mountain. If they proved unable to do so, it would be the proof that the religion of the Christians was wrong and the Caliph would do away with them completely.
And so, after three days of prayers and fasting by the Copts throughout Egypt, Pope Abraam was directed by God to meet someone, St. Simon to mediate for moving the Mokattam Mountain. On the set day, as the people, with St. Simon the Tanner among them, were saying Kyrie eleison (Lord have mercy), the mountain was thrusting up and down, and the sun could be seen from under it. After the miracle was performed in the presence of the Caliph, the Pope turned left and right looking for St. Simon the Tanner, but he had disappeared and no one could find him (a sign of the saint's great humility).
Shortly after the miracle took place, Al-Muizz decided to convert to Christianity. A baptismal font, large enough for the immersion of a grown man, was built for him in the Church of Saint Mercurius. This font still exists today and is known as Maamoudiat Al-Sultan, which means, Baptistry of the Sultan.
In commemoration of this miracle, the Coptic Orthodox Church observes three days of fasting before the beginning of the Nativity Fast (Advent).