If we didn't have a New Testament and were entirely dependent on secular history, the information we would have about Jesus Christ would be very sparse and meagre. Roman historians such as Tacitus (AD 55-120) and Suetonius (AD 69 - 130) mentioned him rather incidentally and only when the Roman Empire was directly involved. Otherwise they were just not interested. Their writings do serve a useful purpose, however. Unsympathetic as they were to the claims, values and beliefs of the Christians, they do at least show that Jesus Christ was a real historical figure and one who exercised some considerable influence on his followers.
When Tacitus mentioned the fire that destroyed Rome in AD 64 for which the emperor Nero had blamed the Christians, he felt he needed to explain who those Christians were. He identified them as a sect that had originated from their founder leader Christ, a man who had suffered the ultimate penalty of death when Pontius Pilate had been the procurator of Judea in the reign of the emperor Tiberius. Tacitus tainted the members of the sect as those who were hated by the general public and the movement itself as a bad superstition which though initially checked had broken out again in Judea, and had even spread to Rome.
Suetonius, a younger contemporary of Tacitus, served as a court official under the emperor Hadrian. When he recorded the expulsion of the Christian Jews from Rome he described them as a group who had been a source of constant disturbance at the instigation of Christ.
Pliny the Younger was the Roman governor of Bithynia in Asia Minor. In AD 112 he wrote to the emperor Trajan to clarify what lawful action should be taken against the large number of those who were being accused of being Christians. He described them as a group that met regularly on a fixed day before it was light, when they sang a hymn to Christ as if he were a god, made their vows to abstain from fraud, theft, adultery and false witness, and then ate some food together.
These records by the Romans were enhanced by certain Jewish and Greek writers. The first century Jewish historian, Josephus, mentioned Jesus as a person in history as did some Jewish rabbinical writings that were put together in the Babylonian Talmud. And Lucian of Samosata, a second century Greek satirist, wrote that the early Christians worshipped the man who had been crucified. They lived by his laws, treated one another as brothers, and denied the gods of Greece.
This is all very well as far it goes. But our debt is to the writers of the New Testament whose work at the least stands as a bloc of ancient literature that sets out to describe and interpret the events surrounding the advent of one whom they present as no less a personage than the promised Messiah, the Son of God.
These writers have presented us with a fuller picture. They wrote down all that they considered was necessary for their readers to know.