Some mosques have Arabic calligraphy and Qur ` anic verses on the walls to help worshipers focus on the beauty of Islam and its holiest book , the Qur ` an , as well as for decoration . Some mosques have Arabic calligraphy and Qur ` anic verses on the walls to assist worshippers in focusing on the beauty of Islam and its holiest book , the Qur ` an , as well as for decoration .