Why are Paul's writings in the Bible?

[Bucky Ball]

So all there is, is your "consensus of scholars". They are no doubt, all "Christians". 
What non-Christians *scholars* studied an historical Paul ?
Are there any historical mentions of a "Paul" or "Saul" in any secular sources ?

Historical Paul is the mainstream scholarly view among basically all NT scholars, Christian or atheist or otherwise. Unless you think people like Bart Ehrman and Richard Carrier are Christians now? Who are the NT scholars that don't believe Paul was a historical figure?

And why the stock argument from ignorance? Should Paul/Saul have been mentioned in any secular source at the time?

Prove it. Yes he should have. Philo says nothing about him. They claimed a student of Gamaliel the Great "converted", yet no one in Judaism mentioned it. They claimed that a Jew (who had no authority to do anything to anyone in Syria) "persecuted" Christians in Damascus, (who were still Jews). At the end of the First Century the Jewish High Priest (in exile) required the recitation of the Expulsion Curses. They (the Christians were still Jews). In the year 400, John Chrysostom told HIS congregation (Istanbul) to STOP going to the synagogue, (Christmas Sermon). There is no coherent "history" of the "church" , and its separation from Judaism. I simply don't buy any of it.