CLAIM: Why does Paul cite this passage in Isaiah 40 to support our need for the Holy Spirit to have wisdom?
RESPONSE: Isaiah 40 is about the infinitude and greatness of God. God knows how much the mountains weigh, and he measured the dust of the earth (Isa. 40:12). The nations are like “a drop from a bucket” compared to him (Isa. 40:15). In other words, Isaiah is stating that God is an infinite-knower, and Isaiah 40:13 comes in the context of our need for revelation. Here, in 1 Corinthians 2:16, Paul is making a case for the need for revelation based on this passage.
On a side note, Morris writes, “Notice that the question in Isaiah 40:13 refers to the mind of Yahweh. But Paul moves easily to the mind of Christ, so closely does he associate the two.”
 Morris, L. (1985). 1 Corinthians: an introduction and commentary (Vol. 7, p. 65). Downers Grove, IL: InterVarsity Press.