If it is true that...

Christianity was immediately extremely controversial

Then it must be true that...

There existed many contemporary anti-Christians

If it is true that...

Contemporary Jewish sources derided Jesus and called him a lunatic

Then it must be true that...

There existed many contemporary anti-Christians

If it is true that...

We lack any contemporary anti-Christian sources that claim that Jesus never existed

and

It would have been a great argument for contemporary anti-Christians to claim that Jesus never existed

and

There existed many contemporary anti-Christians

Then it must be true that...

The historical existence of Jesus is quite certain