The book of James contains doctrinal truths for the Jews living in the last days of the tribulation as they will be scattered and will face all kinds of trouble.
1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.
The term twelve tribes is used all throughout Scripture, and is always a reference to the nation of Israel as a whole and never to a called out group from that nation. Genesis 49:28, Exodus 24:4, 28:21, Ezekiel 47:13. And the NT is no different: Matt 19:28, Luke 22:30, Acts 26:7, Revelation 21:12. There is no justification for taking this as anything other than an address to the twelve tribes of Israel. In order to make James a Christian epistle would mean to contradict every other usage of this term in Scripture. It is certainly not a reference to the diaspora.
If James sent a letter to the twelve tribes of Israel then it has to do with Christianity that they turn to Christianity, or are already Christian, like Hebrews is written to the Hebrews but it explains to embrace Christianity.
Jas 1:1 James, a servant of God and of the Lord Jesus Christ, to the twelve tribes which are scattered abroad, greeting.
Jas 1:2 My brethren, count it all joy when ye fall into divers temptations;
Jas 1:3 Knowing this, that the trying of your faith worketh patience.
Jas 1:4 But let patience have her perfect work, that ye may be perfect and entire, wanting nothing.
Jas 1:5 If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.
Jas 1:6 But let him ask in faith, nothing wavering. For he that wavereth is like a wave of the sea driven with the wind and tossed.
Jas 1:7 For let not that man think that he shall receive any thing of the Lord.
Jas 1:12 Blessed is the man that endureth temptation: for when he is tried, he shall receive the crown of life, which the Lord hath promised to them that love him.
Jas 2:1 My brethren, have not the faith of our Lord Jesus Christ, the Lord of glory, with respect of persons.
Jas 2:6 But ye have despised the poor. Do not rich men oppress you, and draw you before the judgment seats?
Jas 2:7 Do not they blaspheme that worthy name by the which ye are called?
James is speaking to Jews who are Christians which then it would apply to all Christians what he wrote in the book for all Christians go by the same standard.
If James says we are justified by works, and not faith alone, and he is speaking to Jews who are Christians then it would apply to all Christians.
Why would Jews who are Christians have to abide by we are justified by works, and not faith alone, and not Gentiles, when all Christians go by the same standard, and in the Lord there is no difference between the Jew and the Greek, and God made the Jews and Gentiles one man.
Should Jews that are Christians still go by circumcision, and Old Testament ordinances, but they should not.
There is no law for the Jew and one for the Gentile when they are Christians, and if James is writing to Jewish Christians then all Christians have to abide by that.