Excessive Similarities: Paul spoke and wrote on the same themes often, to many different audiences. 49: Paul attends the Jerusalem conference (Acts 15:1-29, Gal 2:2-10). It would not be surprising if the same wording came out from time to time, but with changes to match the different circumstances. 47-48: Paul completes his first missionary journey (Acts 13-14). The author would still be considered Paul, because he decides the actual content and themes of the letter, though not necessarily every word. Theological: It is plausible that Paul could use words in different ways in different contexts. 49: Paul begins his second missionary journey (Acts -). He spent the rest of his life as a traveling missionary, spreading the gospel (good news) to the world; and his letters were part of this. 55: This letter was written after 1 Corinthians, based on the internal references and logical consistency between 1 and 2 Corinthians. Generally, the imprisonment in Rome is considered the most probable (A. However, the case is not airtight, so it is possible that some or all of the captivity epistles were written during some other imprisonment at some other time. Because of his writings, we know more about him than any other person mentioned in the New Testament; but there are still notable gaps. 54-55: Dating is based on persons' names and cross references with 1 Corinthians and Acts. It is likely that 2 Corinthians is actually several letters, combined together, which complicates the issue. The author uses theological terms differently or emphasizes different points. 55: Paul completes the third missionary journey and arrives in Jerusalem (Acts 20-21).
The external attestation is both wide (many sources) and ancient. It is impossible to prove that this Spanish trip did or did not happen because of the thinness of the record. It is likely that Paul did not write many letters until the beginning of his second missionary journey in A. The oldest letter that we have is 1 Thessalonians, and it was written around A. For the full arguments, see the introductions mentioned above. 49-51: Dating is based on traveling companions and cross references between 1 Thessalonians and Acts. This is complicated because there are two possible definitions for Galatia. 52-54: Based on the themes of the letter, it was likely written after 1 Thessalonians and before Paul's next visit to Thessalonica (in Macedonia) in A.
Anachronisms: Concepts are discussed and heresies are argued against that could not have been issues in Paul's time.
Counter-arguments: Because the argument from tradition is so strong, arguments against Pauline authorship should only be accepted if they are overwhelming. In every case, we can give reasons for how Paul could have written the letter in question, in spite of the objections.
Stylistic: When writing a different letter, Paul was writing on a different topic, in a different emotional state, to a different audience, and at a different stage in his life. 52: Paul begins his third missionary journey (Acts -19:1).
Each of these will cause stylistic changes of varying degrees that are very difficult to quantify.