Why are Paul's writings in the Bible?

[GrandizerII]

Too many historical factors to take into consideration, but roughly speaking it partly has to do with Paul being one of the prominent leaders among the mid-first century Christians, and had quite a number of epistles written in his name - some of which he himself didn't write but nevertheless were accepted/believed as such by later Christians.

Which Paul, there are four distinct writers of the Pauline epistles.

The author of Romans.

Four? I thought it was three? Unless you're counting Hebrews as a Pauline epistle.