Secular historians even Bart Ehrman, who wrote bestsellers against the credibility of the New Testament conceded Jesus and His death are historical.
The Gospel of Mark was written no later than 75 A.D. Some argue it was written as early as 40 A.D. Paul’s writings can be dated from 50 A.D. John’s Gospel which is believed as the last gospel to be written was also penned around 95 AD. A small card containing two verses of this Gospel is found in Alexandria which is dated around 130 AD.
Now, compare this to other historical records. Alexander’s history was written 400 years after his death, yet no one questions about its credibility.
Moreover, the martyrdom of the early Christians tells us that they were not lying about Jesus and His resurrection.
Coming to your second question, why did Romans want to deify a person who is said have existed and executed according to that myth in the meanest way? It was after 300 when Christianity was first allowed to be practiced in Roman empire. Before that Christians lived under the persecution of the Roman emperors.
Moreover, Jews don’t say Jesus never existed. In their Talmud tradition, it is mentioned that Jesus practiced witchcraft. They also couldn’t accept the crucified Jesus as their Messiah. Though their perspectives are against Christian belief, it has nothing to invalidate the integrity of the New Testament which of course is the most reliable historical document.
“The Case For Christ” by Lee Strobel" and “More Than a Carpenter” by Josh McDowell" whichI found very helpful in this topic will also help you.
You can also visit https://www.reasonablefaith.org
for further information.