The artist added these saints into his later panels for his north door series having covered the life of Jesus Christ in full and still having around 8-10 panels still to fill. Luke, Matthew, Mark and John also fit together as a series within Christian literature, though opinions and reports of their lives can differ between different religious scripture.
The involvement of God the Almighty is infered by the angel that flies in from the top right hand side of the composition and again Ghiberti uses some of his mastery of architectural drawing to put together Mark's highly elaborate desk display.
Ghiberti took over twenty years to complete his work for these bronze doors and the actual commission was extended several times in order to allow him to achieve the high standards that he desired. Despite the assistance of several members of his burgeoning studio, he was drawn on to other projects from time to time and this delayed things further. Of course, the main reason for such a long lead time on each panel was simply the amount of workmanship that he desired for each and every last detail.
Much of the style used in this series comes from Pisano's earlier doors, which used a Late Gothic approach which Ghiberti both respected and also felt necessary to follow for the sake of consistency.