Why a historical Jesus probably never existed.
There is no contemporary historical record of any kind, of Jesus!! No written Roman, Greek or Jewish sources from this time know of any historical Jesus or Christ. The name "Christ" is mentioned in some later texts (Tacitus, Suetonius Pliny d.y.) but then merely as the name of the idol of the Christians' worship (Read what these sources really say here). We don't even know who the writers of the Gospels were, and don't have the original manuscripts themselves either. We just have later copies of copies of copies of copies … of copies of the assumed lost originals. And with each copy the copyist usually felt free to alter details or rewrite whole parts of the manuscript. (We usually don't trust dubious anonymous sources as evidence for anything, do we?)
All the divine aspects of the Jesus figure are "stolen" from earlier similar dying and resurrected godmen, such as Dionysos, Osiris, Hercules, Attis, Mithra, Horus, Zarathustra and others. Actually there are few (if any) things about Jesus that are original at all. Jesus is just the Jewish version of this popular mythic Saviour- character in the Mystery-religions of Antiquity. (See the similarities here).
All the teachings of Jesus are "borrowed" from older sources, for example from the teachings of Buddha. Many of Jesus teachings are almost word for word identical with some of Buddhas sayings (400 years earlier). The so-called "Golden rule" can be found in several earlier pagan Greek (and Jewish) texts. The famous "Sermon on the Mount" was never held by Jesus, it was actually first produced in the second century AD by Christian priests, assembled from what they assumed were sayings of Jesus in different other texts.
The "birthday" of Jesus is of course unknown, not even the year of his miraculous birth is known. The church just stole the already popular date of the 25th December, which in Antiquity was an immensely popular celebration of the birth of the sungod Mithra, - "the light of the world".
The story of Jesus was originally an allegorical story based partly on the Jewish exodus myth and Joshua/Jesus ben Nun, successor of Moses, the Jewish Messiah-myth and the widespread pagan myth of the dying and resurrected godman Dionysos-Osiris. Later uneducated Christians in Rome, people without the insight and understanding of the deeper meaning of the texts, started to take these allegorical stories for their face value, and Literary Christianity as we know it was born.