“With God, there is “no variation or shifting shadow.” The language is evocative and difficult (see the fourth additional note on 1:17), but the main point is clear: God is unchangeable and entirely trustworthy (cf. Mal. 3:6). Ordinary heavenly lights produce shadows, but God’s light is shadowless. Ordinary lights wax and wane, shift, move around, go through phases, and fade; even the sun is occasionally eclipsed. And all such lights cast shadows that are both inconstant and evanescent. Several heavenly lights are also known in Greek literature as “wanderers” (πλανῆται, planētai; Plato, Leg. 821b; Aristotle, Mund. 392a 13; cf. Jude 13), but God cannot be made to wander, nor does he entice people to wander. God’s light is unchanging; it is not subject to variation. Unlike the sun and moon, God’s light cannot be overshadowed, nor does it make “shifting shadows” (cf. 1 John 1:5: “God is light, and in him is no darkness at all”). The literalistic but memorable turn of phrase “no . . . shadow of turning” used in the KJV, even if not an exact representation of the semantic equivalent of the metaphor that James uses, captures the notion of God’s faithfulness and steadiness. Given the instability of the world in which the nascent community of believers lived, the solidity and reliability of the wisdom of God was important, and the steadiness of the believers as lights is an important corollary in demonstrating that divine wisdom to the world.” -Dan McCartney, James
A Pilgrim's Theology 