Part of the ongoing Verse Quick Reference project.

Isa 40:28 Have you not known? Have you not heard? The LORD is the everlasting God, the Creator of the ends of the earth. He does not faint or grow weary; his understanding is unsearchable.

Isaiah 40:28 is used in several ways by proponents of Classical Theism. The phrase “[God’s] understanding is unsearchable” is often used as a prooftext for omniscience. The phrase “[God] does not faint or grow weary” is used to trump other verses which describe God’s mental exhaustion at dealing with unrepentant Israel (e.g. Jer 15:6). This is not a prooftext for either use.

The context of Isaiah 40:28 has God imparting His regenerating power to those who follow Him. Those who reject God grow weary. Those who accept God never grow weary.

Isa 40:29 He gives power to the faint, and to him who has no might he increases strength.
Isa 40:30 Even youths shall faint and be weary, and young men shall fall exhausted;
Isa 40:31 but they who wait for the LORD shall renew their strength; they shall mount up with wings like eagles; they shall run and not be weary; they shall walk and not faint.

The idea is about being tired from fatigue. This is about power and strength. This is not about mental stress due to trying interpersonal relationships with loved ones, as in Jeremiah 15:6. Isaiah 40:28 is a claim that no one person can overpower God. The verse it is used to counter is about God’s frustration with dealing with people. Both verses (Jer 15:6 and Isa 40:28) can be true in their contexts without one overriding another.

Likewise, “understanding is unsearchable” is not quite a prooftext for total omniscience of all future events. The words being used don’t even necessarily have anything to do with knowledge, but with ability. “Understanding” is contrasted to “knowledge” in various verses and tends to have more in common with “craftiness”, “cleverness”, and even “skillfulness”. Take Psalms 78:72, for example:

Psa 78:72 With upright heart he shepherded them and guided them with his skillful hand.

The same word is translated “skillful”. A translation dealing with “knowledge” might work in this verse, but might not communicate the full intent. In any case, one would be hard pressed to equate “unsearchable understanding” to with modern concepts of omniscience. Nothing in the context suggests that this is what the author intended. Absolutely nothing in the context suggest this needs to extended to complete omniscience of all future events.