Why do we refer to our first name as our Christian name?
Originally, ”Christian” names were names given in recognition of the Christian upbringing or Christian conversion of the person named.
In the early centuries of Christianity, it seems it was rare for a person who had become a Christian to change their name – Saul to Paul is an example, though Saul continued to be used of him.
Early “Christian names” which were given to children might be the names of Old Testament characters and New Testament Characters (which apparently took longer to become fashionable).
Later on children were often named after Christian teachings (eg. Redemptus), Christian Festivals (Sabbatus), Christian Virtues (Agapetus), and Christian Sentiments (Hilarius). Which I find truly hilarious.
Still later, children were named after famous Christians or saints from previous times. They were usually famous because they had been martyred! This gave names like Agnes and Irenaeus. Often the saint chosen was the one on whose feast day the child was born.
Occasionally, churches, especially the Roman Catholic church, would issue decrees which encouraged the faithful only to use Biblical names, but these were widely ignored, and by the middle ages, there were as many names given without any Christian associations as with.
A study of the registers of Oxford University from 1560 to 1621 produced this list of the more common names in order of popularity: John, 3826; Thomas 2777; William, 2546; Richard, 1691; Robert, 1222; Edward, 957; Henry, 908; George, 647; Francis, 447; James, 424; Nicholas, 326; Edmund, 298.
Apart from the fact they were all male, this list could well have been made recently! Notice the small number of Bible names, though those which are present seem popular.
Does this help?