How many letters (epistles) are in New Testament? What are the purpose of those letters?

There are 22 letters (epistles) in New testament. 14 of them were written by
St Paul   to church and it leaders. Jesus's apostles St peter, St John, St Jude and St James also wrote letters to believers emphasizing various matters associated with early church. 

Epistles are catogorized as:

1. St Paul ( 14 letters )
2. St Peter (2 letters )
3. St John ( 4 letters - 3 letters and revelation )
4. St James (1 letter )
5. St Jude ( 1 letter )

Letters and its purpose are given below:

By St Paul

Letters	Purpose
St Pauls' Letter to the Romans	To address the division between the Jewish and gentile Believers
St Pauls' first Letter to the Corinthians	To help Christian growth, enlightening the church about spiritual gifts and to prepare for the second coming of Jesus
St Pauls' second Letter to the Corinthians	To defend his position as the Apostle to the Gentiles and to encourage the Corinthians Church support
St Pauls' Letter to the Galatians	To address those causing division by emphasizing Jesus and circumcision over salvation through Christ alone.
St Pauls' Letter to the Ephesians	to enable beleivers to understand our spiritual position in Christ.
St Pauls' Letter to the Philippians	To thank the believers for their support
St Pauls' Letter to the Colossians	To encourage an accurate view of Christ as the creator of all things
St Pauls' Letter to the Thessalonians	To encourage the Believers to stay strong through suffering
St Pauls' second Letter to the Thessalonians	Purpose
St Pauls' Letter	To remind Christians of the second coming of Jesus and how to prepare for it
St Pauls' Letter to Timothy	To Timothy in regards to his leadership of the Church in Ephesus
St Pauls' Letter to the Timothy	To encourage Timothy to walk in Gods' calling!
St Pauls' Letter to Titus	To instruct Titus of the governance to the Church in Crete
St Pauls' Letter to Philemon	To a friend about accepting a previous slave as a brother
St Pauls' Letter to the Hebrews	To illuminate the superiority of the sacrifice of Jesus over that of the Old Covenant priesthood

Other letters in New testament


By St James

Letters	Purpose
St James' Letter to Jewish Believers	To emphasize right and just living

By St Peter

Letters	Purpose
St Peter' First letter to believers	To instruct Believers about spiritual growth and their position in God
St Peter' Second letter to believers	To prepare believers for the second coming of Christ and avoid the abuse of Grace

By St John

Letters	Purpose
St John' First letter to believers	To help Believers know God and realise who they are in Him
St John' Second letter to believers	To help Believers avoid becoming a partakers of ungodly actions
St John' Third letter to believers	To draw their attention to wickedness of Diotrephes
Book of Revelation	Detailed prophetic visions , details of end of days and second coming of Christ.

By St Jude

Letters	Purpose
St Judes' Letter to Jewish Believers	As a warnings to believers from abusing the Grace of God and falling into sin

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