Every reference throughout Scripture is always to the entire nation of Israel. The term twelve tribes is never used for Christians. Notice it does not say to the Jewish Christians out of the twelve tribes, but simply the twelve tribes.
1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.
When will the twelve tribes of Israel be scattered abroad? Many prophetic references in the OT point to Jacob's trouble, the tribulation.