Celtic Literature and Illuminations. Celtic scholars had a great influence on ninth- and tenth-century philosophy and theology. Celtic literature and the decorative techniques used in their manuscripts also influenced European literature, particularly epics and other poetry. Although the stories and themes of this literature were rooted in the pre- Christian era, much of it was written down in the period after 814. In fact, one of the distinguishing characteristics of Celtic literature is its blend of pagan and Christian themes, especially the presence of divinity in nature and a sense of the supernatural. The Celtic hero is often visited by benign and malignant spirits and is susceptible to visions. The voyage across a long, deep sea or to another world is another Celtic motif that had a major influence on later medieval literature. Poems such as the Tain (first written...