Charles Ellicott Commentary James 4:15

"For that ye ought to say, If the Lord will, we shall both live, and do this or that." — James 4:15 (ASV)

For you ought to say...—Referring to James 4:13, in such a continuation of reproof as this: Woe to you who say, ... instead of saying, “If the Lord will”... In fact, it refers to a completed action on God’s part—past in the sense of being divinely accomplished, not merely past in time—“If the Lord has willed it, we will both live and do this or that.” Such an attitude, it should be noted, is far from fatalism, even in its best form, such as under the teaching of Islam.

The sovereignty of God is acknowledged, but with it is plainly recognized the existence of man’s free will, dependent, however, on the permission of the Most High for its fleeting duration and power. Saint Paul speaks in a similar tone of coming to Corinth, if the Lord will (1 Corinthians 4:19); and the expression “God willing” (short for the Latin Deo Volente) is used, reflecting the conviction that “the reference of all the contingencies of the future to One supremely wise and loving Will, has been in all ages of Christendom the stay and strength of devout souls.”