Thomas appears to have been Gilbert and Matilda’s only surviving son. They also had at least three daughters who survived into adulthood. Two of those daughters, Agnes and Roheise, married, though their husbands’ names are not certain, and both had children. In the 1160s, they and their husbands and children suffered greatly at the hands of Henry II because of his anger at Thomas. The third daughter, Mary, entered a nunnery; in 1173, King Henry II appointed her abbess of Barking Abbey. It is possible that there was a fourth daughter who was the mother of Agnes’s eventual heir, Theobald of Helles (or Hulles), because Theobald called himself Saint Thomas’s nephew but did not refer to Agnes as his mother. All told, Gilbert Becket had at least seven grandsons, two of whom became priests, and two great-grandsons.
Thomas was born on December 21, probably in either 1118 or 1120. December 21 was the feast day of Saint Thomas the Apostle, after whom the future archbishop was probably named. Indeed, it is likely that it was the fame of Saint Thomas of Canterbury that popularized the name Thomas among the English, for the name was not common in England or Normandy before the late twelfth century. Little is known about Thomas’s early life. His biographers, even the earliest ones, are interested primarily in his career as archbishop and tell us little about anything before 1162, and medieval sources about the lives of children, even the children of monarchs, are usually sparse and uninformative. There are a few stories, typical of the lives of saints, about his mother’s dreams of his future greatness and about events in his early childhood that prefigured it.