Jamieson, Fausset and Brown on James 1:9-10

9-10. Translate, "But let the brother," &c. that is, the best remedy against double-mindedness is that Christian simplicity of spirit whereby the "brother," low in outward circumstances, may "rejoice" (answering to Jas 1:2) "in that he is exalted," namely, by being accounted a son and heir of God, his very sufferings being a pledge of his coming glory and crown (Jas 1:12), and the rich may rejoice "in that he is made low," by being stripped of his goods for Christ's sake [Menochius]; or in that he is made, by sanctified trials, lowly in spirit, which is true matter for rejoicing [Gomarus]. The design of the Epistle is to reduce all things to an equable footing (Jas 2:1; 5:13). The "low," rather than the "rich," is here called "the brother" [Bengel].