While the New Testament is by far the most carefully and accurately preserved document of its era, Jesus' existence is also attested to by historical sources outside the Bible. Here are some:
Thallus (Samaritan historian, ca. A.D. 52):
Attempted to give a natural explanation for the darkness which occurred at Jesus’ crucifixion. He didn’t deny the existence of Jesus - only tried to explain strange circumstances surrounding His death.
Mara-Serapion (written to his son, ca. A.D. 73):
Tells of the deaths of Socrates, Pythagoras, and Jesus; he writes of Jesus: “What advantage did the Jews gain from executing their wise king?”