Arab imagery
Omar Ibn Said's practice of Arabic calligraphy is an invocation of Arab imagery, signaling the reader to sympathize with his homelands. He wasn't abducted from nowhere; he had a fully formed identity in his homelands, and so the subtle reminders of his former identity are haunting imagery, designed to show the devastation of his abduction, and the violence of his removal from home.
American imagery
The setting of the memoir is America, so the imagery he describes helps the reader understand that to him, American life, nature, and culture are not self-explanatory. He has to adapt and learn, but to him America is a strange land that he is not suited for, necessarily. The American imagery includes descriptions of society and nature, because both differ greatly from what he's used to.
Christian imagery
Omar Ibn Said adopts a Christian opinion when he encounters the beauty of the Christian story. He doesn't resonate with Christians necessarily, because he sees that objectively, they struggle with serious hypocrisy in many cases; they endorse the mistreatment of slaves, for instance. They often make enemies, which is contrary to Christ's teaching. But Omar notices this because he quickly learns that Christianity is not exactly what the Christians show him. He encounters Christianity through churches, through the Bible, through the culture, and in many other ways that are more subtle.
Islamic imagery
Although historically, this aspect of the memoir is frequently ignored, Omar doesn't abandon Islam to become Christian. Instead, he maintains a quiet, steady attachment to the truth he encountered through his Muslim identity, and he even recites the Koran. In fact, one might even suggest that he himself is a portrait of a Muslim living under extreme duress. When he remembers the culture of his homelands, he reminisces about the Islamic culture there.
