The modern form of taking an oath is by placing the hands on the Gospels or on the Bible. The corporate, or corporal both, is the name of the linen cloth on which, in the Roman Catholic Church, the sacred elements consecrated as “the body of our Lord” are placed. Hence the expression corporal oath originated in the ancient custom of swearing while touching the corporal cloth. Relics were sometimes made use of. The laws of the Allemanni (chapter 657), direct that he who swears shall place his hand upon the coffer containing the relics. The idea being that something sacred must be touched by the hand of the jurator to give validity to the oath, in time the custom was adopted of substituting the holy Gospels for the corporal cloth or the relics, though the same title was retained.
Haydn (Dictionary ok Dates) says that the practice of swearing on the Gospels prevailed in England as early as 528 A.D. The laws of the Lombards repeatedly mention the custom of swearing on the Gospels. The sanction of the church was given at an early period to the usage. Thus, in the history of the Council of Constantinople, 381 A.D., it is stated that “George, the well-beloved of God, a Deacon and Keeper of the Records, having touched the Holy Gospels of God, swore in this manner,” etc. A similar practice was adopted at the Council of Alice, fifty-six years before. The custom of swearing on the Book, thereby meaning the Gospels, was adopted by the Medieval Gild of Freemasons, and allusions to it are found in all the Old Constitutions. Thus in the York Manuscript, No. 1, about the year 1600, it is said, “These charges . . . you shall well and truly keep to your power; so help you God and by the contents of that Book.” And in the Grand Lodge Manuscript No. 1, in 1583 we find this: “These charges ye shall keep, so help you God, and your haly dome and by this book in your hand unto your power.” The form of the ceremony required that the corporal oath should be taken with both hands on the book, or with one hand, and then always the right hand.
The practice of kissing the book, which became so well established in England, appears in the Middle Ages (see J. E. Tyler, Oaths, pages 119 and 151).
the source: Mackey’s Encyclopedia of Freemasonry