Four main sources
Most traditional English first names come from four wells. Hebrew, through the Bible: John, Mary, Michael, Elizabeth, Sarah, Daniel. Greek and Latin: Alexander, Sophie, Julia, Marcus, Helen, Lucy. Germanic, brought by Anglo-Saxon and Norman settlement: William, Robert, Richard, Edward, Matilda. And Celtic, from Ireland, Scotland and Wales: Brian, Sian, Ciara, Rhys, Morgan, Fiona.
Many names began as ordinary words: Sophie means "wisdom" in Greek, William means roughly "resolute protector" in Germanic, Ciara means "dark-haired" in Irish. That meaning has no effect whatsoever on the person who bears it — but it usually explains why the name was given for centuries.
Saints, kings and the Bible
For most of the last millennium, naming was highly constrained: children were called after saints, monarchs, godparents or grandparents. That is why the stock of names stayed so small — a handful of names covered most of the population, and an English village could be full of Johns and Marys.
The Reformation widened the pool in Protestant countries by bringing Old Testament names into common use, and the Victorians revived flower and virtue names — Rose, Lily, Grace, Hope — many of which are back in fashion today.
Fashion moves in measurable cycles
Diversity has exploded in the last century: the most popular name once accounted for a large share of births, whereas today the top name typically covers only around 1% of babies in the countries that publish the figures.
Names also follow a strikingly regular cycle: a name emerges, spreads, peaks, then declines precisely because it has become common — and reappears two or three generations later, once it evokes great-grandparents rather than parents. That is why Arthur, Florence, Alfie, Ivy, Theodore and Evelyn are back.
Check the official data
Most countries publish annual baby name statistics, through the national statistical office or the civil registry; the sources are listed country by country. See name statistics.