Who were the twelve tribes scattered abroad?

Answer:

"James, a bond-servant of God and of the Lord Jesus Christ, To the twelve tribes who are dispersed abroad: Greetings" (James 1:1).

The book of James is written for Jewish Christians who were scattered by persecution (Acts 8:1-4). This is one indication that James is one of the first books written. It would have been written prior to the large conversion of Gentiles that started in Acts 13. It is that persecution of the early church that sets up the theme for the first part of the book.

For those who see James as being written later, they state that the church replaced Israel as God's people (Romans 2:28-29), so the "twelve tribes" is just a figurative way of referring to the church. Thus, the scattering could refer to any number of persecutions by either the Jews or the Romans.