Once, in the long ago times, pagans inhabited the world. Pagans were pretty much at ease with the idea that whomever or whatever you held a sacred was ok; there was plenty of space in the world for a pantheon of deity and still enough room to raise sheep.
However, pagans were, as a whole, a fairly unimaginative group and as such had a difficult time coming up with one name for each person, let alone two. Most of the time people were called by their trade (such as Miller, Smith or Baker) and on occasion named for some small peccadillo that they may have thus names like “Ramsbottom” and “Goldenshower”.
Then, slowly, crawling and slinking out of the muck and filth, a religion formed that demanded fealty to a single god and this was not only required of the faithful but of every other human inhabitant of the planet. When they slithered into a town they took over because the pagans were like, “Whatever dude, you do what makes you happy”. While this attitude worked with other pagans, it turned out to be a rather self-defeating strategy when applied to a group that wanted to control everything and everyone.
But, Christians could not just rename the people whose lives they wanted to take over; no one would know who the hell was being called, spoken to or brought to trial on heresy charges. So, they implemented a system by which they added a second name to each person so there could be some way to differentiate between the various Ramsbottoms so everyone would know who was the actual blasphemer.
Now, being bossy but somewhat insecure, they decided that this additional random name would not be “added” but would be said first and then followed by the Surname (so called because common pagans got SURly when called out by Christians for some minor transgression). Over the intervening years, this first name came to be called the “Christian Name”; some say in honor of the inventors, some say to spite them but the true reason has been lost in the annals of time.