Tacitus
Tacitus was a Roman senator and the greatest historian of the Roman Empire. He had no time for Christians. He called their faith a "most mischievous superstition."
Writing around AD 116 about the great fire of Rome, he explained who the Christians were:
"Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus."
Every detail matches the Gospels. The name Christ. Executed, the extreme penalty. In the reign of Tiberius. By Pontius Pilate.
A hostile Roman historian, confirming the crucifixion from Rome's own side.
Josephus
Josephus was a Jewish priest and general who went over to the Romans and wrote the history of his people. Around AD 93, he names Jesus twice.
The first time, he describes the man himself:
"About this time there lived Jesus, a wise man, if indeed one ought to call him a man. For he was one who performed surprising deeds... He was the Christ. And when, upon the accusation of the principal men among us, Pilate had condemned him to a cross, those who had first come to love him did not cease. He appeared to them spending a third day restored to life, for the prophets of God had foretold these things and a thousand other marvels about him."
A Jewish historian, writing for Romans, records that Jesus was crucified under Pilate, and that his followers saw him alive again on the third day.
The second time, he's recording the death of James in Jerusalem, and identifies him like this:
"the brother of Jesus, who was called Christ, whose name was James"
That's the same James the New Testament calls "the brother of the Lord" (Galatians 1:19). Josephus mentions Jesus in passing, as someone his readers would already know.
Pliny
Around AD 112, Pliny the Younger was the Roman governor of a province in what is now Turkey. He wrote to the Emperor Trajan asking what to do about the Christians.
He reported what they did. They met on a fixed day, before dawn, and sang a hymn to Christ as to a god.
Less than a century after the cross, the people Rome was trying to stamp out were worshipping Jesus as God.
Why this is a big deal
These men weren't Christians. Tacitus despised them. Pliny was putting them on trial.
Yet between them, they confirm that Jesus lived, that he was called the Christ, that Pontius Pilate had him executed in the reign of Tiberius, and that his followers worshipped him as God. That is the core of the Gospel story, told by its enemies.