Baptism was called “christening” because it brought the child (or adult) formally into the Christian church, perhaps also bringing the spirit of Christ into the child’s heart. It also gave the child his official Christian name. For an infant born to a Christian family, this just meant being given a name. For medieval pagan Saxons, Danes, Swedes, Huns, or Prussians, it meant being given a “Christian name” in addition to the one already in use. This Christian name usually recalled a saint, including the names in the Bible. For infants in already-converted families, there was a wider choice.
Since christening could take place as late as six weeks after birth, it suggests that the baby had no name at first. There were probably families who really did not name an infant until it had survived for a few weeks. In other cases, a child may have been called by the chosen name from the start, but it was at this ceremony that the name was recorded in the church records. The baptism/christening record served as proof of birth and identity.
European names in the early Middle Ages tended to be the old pre-Christian names from Germanic roots. They were most often two syllables and had been compound words in the original language, even if nobody thought about the root meanings any more. Elements like joy, friend, noble, wolf, bear, spear, guard, elf, counsel, bright, fame and battle were repeated in various combinations.
In the pre-medieval years, as the Roman Empire collapsed, Germanic people settled all over: Goths in Italy, Visigoths in Spain, Franks in France and Germany, Saxons in Germany and then England, Danes and Swedes in Scandinavia. The same naming traditions, using the same root words, went to all of these places. By the Middle Ages, the streams were beginning to meet and mix again. Most obviously, when the Normans invaded England in 1066, they brought both Danish and Frankish names. These Norman names displaced the native ones, but not entirely.
The Anglo-Saxon names of this type included Edwin, Edgar, Alfred, Harold, Wilmer, and many other names no longer familiar (you can read a list here). We don’t use many of the Anglo-Saxon women’s names; it’s a rare exception, like Ethel or Edith, that has been revived. Girls’ names seemed to have switched to Norman forms more strongly than boys’ names.
It’s hard to sort out which names are precisely Frankish, Danish, Visigoth-Spanish, and so on. We can see some trends in, for example, the names of the first wave of Normans: William, Robert, Richard, Henry, Geoffrey, Matilda, Emma, Adelaide. Soon after, we add Reginald, Gilbert, Fulk, and Mabel.
Early kings and queens of the Franks: Pepin, Louis, Charles, Carloman, Lothair, Otto; Hildegard, Fastrada, Luitgard, Bertha, Gisela, Adalhaid (Frankish form of Adelaide).
Early kings in Spain: Ferdinand, Sancho, Alfonso, Garcia, Ramiro, Bermudo.
The First Crusade gives us, in addition: Godfrey, Louis, Baldwin, Raymond, Bohemund, Tancred, Hugh, Alice and Melisande.
All of these names are made from two Germanic roots. Some of the names are no longer in use, like Luitgard and Bohemund, but many of them are still doing well. So these can be considered the foundation layer of European names, already in use before they converted to Christianity.
Certain common names were used over and over during these centuries, and further, some aristocratic families tended to conserve certain names, using them over and over, so that it was often easy to know what family someone came from by his Christian name. Counts of Flanders were generally named Baldwin, while Counts of Toulouse were most often called Raymond. Kings of Castile were Ferdinand or Sancho, over and over. Scattered among these famous aristocratic examples, we can be sure that the common people had similar regional, local and familial alternations between generations.
As the Christian culture grew stronger, babies were more often given names out of the Bible. Boys were John, Thomas, Matthew, Stephen, Peter and Paul. Girls were Joan, Joanna, Mary, Elizabeth and Anne. But another strong naming stream developed from saints: Benedict, Martin, Nicholas; Agnes, Catherine, Margaret, Barbara and Clara. The two naming streams came together as some early converts were declared saints, lending religious value to their pagan-root names: Anselm, Bernard, Bruno, Cuthbert, Edmund, Edward, and Hildegard.
The most popular English men’s names around 1400: Adam, Geoffrey, Gilbert, Henry, Hugh, John, Nicholas, Peter, Ralph, Richard, Robert, Roger, Simon, Thomas, Walter, William. Most popular English women’s names: Agnes, Alice, Avice, Beatrice, Cecily, Emma, Isabella, Joan, Juliana, Margery, Matilda and Rohesia. I have to admit I didn’t see Rohesia coming, but I’ll trust this person’s research: website on medieval names.
Naming traditions are highly regional and tend to be strongly conservative. In medieval Europe, in general, babies were named to honor someone, whether relative, friend or saint. England developed a tradition of naming babies after their parents, first. If John and Joan had a boy and a girl, their names were certainly John and Joan; variety came only with later children.
Some death records of families seem to suggest that if infant John died, his name might be recycled for the next baby boy. There may have been families in which two children shared the same name, though certainly a nickname must have been used to keep them separate. This may have occurred when an infant was named for one of his godparents, and a sibling happened to have a godparent whose name had also been used, and was the same. In other regions, there were traditions of naming babies after the saints on whose days they had been born.