Sinners and double-minded, there are two groups being spoken of here. These statements are not both directed to the saints. Look into it. See the context of the statement and what was going on to whom he is addressing.
Hi Ben,
Consider that man put in the paragraphs, chapters and verses. As we read all of James, his audience is the believers from the 12 tribes who were dispersed. So they are Jewish brethren. Probably they were some of the converts right after the Holy Spirit came upon them on the day of Pentecost where thousands were saved in one day.
As you read James you'll find that in the whole letter he is correcting brethren right and left on multiple issues. They had a lot of things wrong and were in need of correction. The text itself tells us that all of these people were believers (brethren). To say otherwise is to ignore the clear printed word, brethren.
Read James as a whole and without chapters and verses. Maybe that will help you see that James was bringing to the brethren a word from God in obedience to the Great Commission.......go and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit, teaching them to observe all that I commanded you; and lo, I am with you always, even to the end of the age.” Matthew 28:19-20