Anyone that says that Jesus did not exist would be laughed at by any serious historian. The standard scholarly dating, even in very liberal circles, is that the Gospel of Mark was written in the 70s, the Gospel of Matthew and Luke in the 80s, and the Gospel of John in the 90s. That's still within the lifetime of various witnesses of the life of Jesus, who was crucified in the year 30-33 AD, including hostile witnesses who would have served as a corrective if false teachings about Jesus were going around. Furthermore, the book of Acts cannot have been written any later than 62 AD. Since Acts is the second part of the Gospel of Luke, that means that the latter must have been written even earlier than that, and since the Gospel of Luke incorporates part of the Gospel of Mark that means that Mark must have been written even earlier (i.e., earlier than 60 AD). Compare that to the earliest biographies of Alexander the Great, which were written four hundred years after Alexander's death. Yet, I see no one calling the existence of Alexander the Great into question. Finally, Paul's letter were written in the 40's and 50's and incorporate creeds that were already part of the established oral tradition of the church. These creeds, that include the belief in the divinity of Jesus, can be found in Philippians 2:6-11, Colossians 1:15-20 and 1 Corinthians 15.

"For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures, and that he appeared to Cephas, and then to the Twelve. After that, he appeared to more than five hundred of the brothers and sisters at the same time, most of whom are still living, though some have fallen asleep. Then he appeared to James, then to all the apostles, and last of all he appeared to me also, as to one abnormally born." (1 Corinthians 15:3-8)

Hence, we have early written attestations and an oral tradition that traces back to Jesus' very crucifixion. Finally, your claim "not one eyewitness wrote about him (Jesus)" is patently false:

"And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth." (John 1:14)

"And he that saw it bare record, and his record is true: and he knoweth that he saith true, that ye might believe." (John 19:35)

"This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true." (John 21:24)

"For we did not follow cleverly devised tales when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty." (2 Peter 1:16-19)

Matthew, John and Peter were members of the twelve original Apostles with the latter two being personally very close to Jesus. They all were close companions of Jesus in His ministry and they all saw Him alive after His resurrection from the dead. Paul and James saw Jesus alive after His crucifixion and burial. Paul was personally chosen by Jesus to preach the gospel to the non-Jewish Roman world. James was a member of Jesus household as His half brother and a close associate of the Apostles being a prominent member of the early church. Jude was a contemporary of Jesus and the Apostles and was a brother of James and half brother to Jesus who surely qualifies as a reliable witness to the truth. Mark and Luke were contemporaries of Jesus and were both very close associates of the Apostles. Both had a tremendous opportunity through their association with the Apostles and many others to know the facts about Jesus.