In considering the authenticity of any the Pauline letters, it is necessary to first consider what it is that confirms the validity of any of Paul's writing. The style of writing, use of vocabulary and emphasis on a common theme provide the link to the recognized letters of Paul. Worldwide the doctrine, reference to faith and the Mosaic Law provide the necessary confirmation.
The confusion and denial that surrounds the Deutero-Pauline letters (Colossians, II Thessalonians, Ephesians) and the Pastoral letters (Timothy I and II, Titus) is attributed to the reference to unacknowledged beliefs and to style and vocabulary. For example, Paul often refers to "heaven" in Romans, Galatians, Philippians but in Ephesians, he makes several references to "heavenly places."
Colossians and Ephesians are similar to each other in the teaching of dogma which distinguishes them from Paul's traditional teachings. Unmistakable Pauline concepts such as The Holy Spirit, place of the Law and Justification - the removal of Man's sin through Jesus as the Savior- "By the free gift of God's grace all are put right with him through Christ Jesus, who sets them free."(Romans 3:24)- carry different interpretations in Colossians and Ephesians. These differences contributed to the division of the Church and changing emphasis by newer branches of Christianity.
Faith, hope and love - I Corinthians 13.13 confirming "These three remain; faith, hope and love..."- pronounced emphatically by Paul are replaced by references to faith and love in II Thessalonians. There is also an impersonal style in II Thessalonians in contrast to an established relationship between Paul and the people which exists in I Thessalonians. Furthermore, some common words and phrases used by Paul throughout I Thessalonians, carry different interpretations later.
In the Pastoral letters, the same difficulties arise with vocabulary, style and context. The increasing importance placed on "good works" is reinforced. Salvation by faith and the transforming power of The Holy Spirit are the essential messages of Paul's letters to the Romans as..."God's Spirit lives in you."(Romans 8:9) but in Titus 3:5 "He saved us...not because of any good deeds.." he then goes on to "give special emphasis...to doing good deeds.."(Titus 3:8) In the letter to Titus, reference to "in Christ," one of Paul's most frequent phrases is notably absent. In the Pastoral letters, modern references to words not recognized also contribute to the disputed authorship.
Biblical references are from The Good News Bible. Sixth Impression South African Edition. Bible Society of South Africa