Book of Kells (ca. 9th Century CE), "Chi-Rho Page"

Matthew 1:18 begins, in Greek, "XPI autem generatio...." (Douay-Rheims trans.: "Now the generation of Christ was in this wise.When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.")  The prominence of this initial signals the beginning of the incarnation of God in Jesus' lineage.