In ancient times, oil was often used to anoint new kings. A king was therefore known as an “anointed one.”
In Hebrew, “anointed one” is messiah. In time, the great king that was to save the Jews came to be known as The Messiah. In the Greek, this translates as o Christos, the Christ. So the apostles went out and preached about the Christ, the Messiah, the anointed one.
However, some people began to confuse the greek word Christos with a similar greek word, Chrestos (which meant only “good man”). Because of this, it seems that preachers of the Gospel began to drop the “the” and use simply “Christ,” more like a name. This helped listeners to realize that it was not simply a description about some hypothetical good man, but was an actual name for an actual man.
Hence, Jesus the Christ became simply Jesus Christ. (It is also sometimes written as Christ Jesus – which also makes sense.)
Another factor that played into this usage over time was that there is not an equivalent “the” in Latin. So when all of the scripture was translated to Latin and then to other languages, Jesus Christ – without the “the” – was found more often.
And that’s how Jesus of Nazareth, the annointed one, the messiah came to be known, at least namely, as Jesus Christ.