John Calvin Commentary James 1:10

"and the rich, in that he is made low: because as the flower of the grass he shall pass away." — James 1:10 (ASV)

But the rich, in that he is made low, or, in his lowness. He has mentioned the particular for the general, for this admonition applies to all those who excel in honor, dignity, or any other external thing. He tells them to glory in their lowness or littleness, in order to repress the haughtiness of those who are usually inflated with prosperity.

He calls this state lowness because the manifested kingdom of God should lead us to despise the world, once we recognize that all the things we previously greatly admired are either nothing or very insignificant. For Christ, who is a teacher only to the childlike, curbs by His doctrine all the haughtiness of the flesh. Therefore, so that the vain joy of the world does not captivate the rich, they should accustom themselves to glory in the casting down of their fleshly preeminence.

As the flower of the grass. If anyone were to say that James alludes to the words of Isaiah, I would not strongly object. However, I cannot agree that he is quoting the Prophet's testimony, who speaks not only about the things of this life and the transient nature of the world, but about the whole person, both body and soul (Isaiah 40:6–8); in contrast, the subject here is the grandeur of wealth or riches.

And the meaning is that glorying in riches is foolish and preposterous because they pass away in a moment. The philosophers teach the same thing, but this teaching falls on deaf ears until the Lord opens one's ears to hear the truth concerning the eternity of the heavenly kingdom. This is why he mentions brother, implying that this truth cannot be received until we are accepted into the family of God’s children.

Although the commonly accepted reading is ἐν ταῖς πορείαις, I agree with Erasmus and read the last word as πορίαις, without the diphthong: “in his riches,” or, “with his riches”; and I prefer the latter.