The Apostles began showing up on the walls of the catacombs beginning within the 1st and 2nd centuries. in  Early Christian paintings  sacred themes were initially expressed in  visual symbols. The first Christians feared torture or a stint in the arena if they flaunted their religion openly. With the Christianization of Europe  artists of the Migration and Carolingian period depicted Madonna's, The Apostles and Christ in illuminated manuscripts, mosaic and frescos. The Byzantine  era was the Christian art at its zenith. The work of masters such as Duccio di Buoninsegna and Pietro Lorenzetti  became prized by the church and feudal courts for their compositional unity and emotional  Christianized intensity. 

Apostles and Saints were often shown with open books. Saint Augustine (354-430) was regularly depicted sitting at his desk with an open heart-shaped book, symbolic of his passion for truth and reverence for the Word of God, the ultimate  source of light and truth.

 The Spiritual Illuminators of the World

Regarding the life and occupations of the Apostles prior to their life with Jesus , W.D. Killen writes" All the Twelve, when enlisted as disciples of Christ, appear to have moved in the humbler walks of life; and yet we are scarcely warranted in asserting that they were extremely indigent. Peter, the fisherman, pretty plainly indicates that, in regard to worldly circumstances, he had been, to some extent, a loser by obeying the call of Jesus. [39:7] Though James and John were likewise fishermen, the family had at least one little vessel of their own, and they could afford to pay "hired servants" to assist them in their business. [40:1] Matthew acted, in a subordinate capacity, as a collector of imperial tribute; but though the Jews cordially hated a functionary who brought so painfully to their recollection their condition as a conquered people, it is pretty clear that the publican was engaged in a lucrative employment. Zacchaeus, said to have been a "chief among the publicans," [40:2] is represented as a rich man; [40:3] and Matthew, though probably in an inferior station, was able to give an entertainment in his own house to a numerous company. [40:4] Still, however, the Twelve, as a body, were qualified, neither by their education nor their habits, for acting as popular instructors; and had the gospel been a device of human wisdom, it could not have been promoted by their advocacy. Individuals who had hitherto been occupied in tilling the land, in fishing, and in mending nets, or in sitting at the receipt of custom, could not have been expected to make any great impression as ecclesiastical reformers. Their position in society gave them no influence; their natural talents were not particularly brilliant; and even their dialect betokened their connection with a district from which nothing good or great was anticipated. [40:5] But God exalted these men of low degree, and made them the spiritual illuminators of the world."