Modern analytical bible scholars have intensively studied the text of those epistles that are generally attributed to Paul. By closely examining vocabulary, grammar and thought themes they are in agreement that the following epistles are genuinely from Paul. They are 1 Thessalonians, 1 and 2 Corinthians, Galatians, Philippians, Philemon and Romans.
Two more letters, Colossians and 2 Thessalonians are in dispute.
Hebrews does not reflect Paul’s style and content whatsoever.
Ephesians does not reflect the style of Paul but is very much Pauline in content and is thought to have been written by a close follower of Paul’s.
The Pastoral letters (Titus, 1 Timothy and 2 Timothy) are attributed to Paul, but someone writing in Paul’s name wrote them around AD120, some 60 years after Paul’s death.
Each letter uses vocabulary Paul is not known to have used; each has a different concept than Paul had of key matters such as faith; and each refers to Paul’s close friends Timothy and Titus in formal rather than friendly terms. They assume that Christian churches are governed by the kind of carefully organized authority structures that developed decades after Paul’s time. They are similar in style and in content and in the issues they raise. Scholars generally believe them to have been written by the same person. In addition two of Paul’s epistles are thought to be composed of what were originally several smaller letters. In particular Philippians is composed of three and 2 Corinthians is composed of six. Chapter 16 of Romans seems to be a later addition but genuinely by Paul.