The General Epistle of James, written by the brother of our Lord, provides ethical instruction for all Jewish people who have become Christians. It is clear and practical in its dealing with Christian behavior.

James 4

4:11 Speak not evil one of another, brethren. He that speaketh evil of his brother, and judgeth his brother, speaketh evil of the law, and judgeth the law: but if thou judge the law, thou art not a doer of the law, but a judge.