for you and those you mislead.
The historical record for Jesus is far stronger than it is for numerous other historical figures you wouldn't think of dismissing as fiction.
You have quite the double standard in that regard.
If the early Christians wanted to take over any pagan winter solstice feast, why did they miss the actual winter solstice? Their astronomy was better than to miss it by several days.
https://www.patheos.com/blogs/standingonmyhead/2018/12/yes-jesus-really-existed-and-he-was-born-on-december-25.html?utm_medium=social&utm_source=share_bar
Oh yes, the other detail is that there are records that Christmas was celebrated on December 25 from the time of St Telephorus–the seventh pope who was born in 115 AD. The cult of the birth of Sol Invictus was not established until 274 AD, so if anybody was copying celebrations, it is more likely that the Roman Emperor Aurelian was copying the already existing feast of Christmas–the birthday of Christ the Unconquered Son of God–than the other way around.
The bible itself is considered a good history source, but there are also quite a few extrabiblical sources as well:
Did Jesus Christ Really Exist? Proving Jesus Without the Bible
https://beginningandend.com/jesus-exist-historical-evidence-jesus-christ/
and of course there were all of those Christian martyrs who were persecuted by Rome & refused to recant their belief in Jesus even under penalty of being fed to the lions, crucified, burned at the stake etc.
Despite the Jews and the Romans, the most powerful nation on earth by far, persecuting them the Christians persevered, splitting time in half... BC and AD.