Paul uses an Old Testament question to close his argument. Barnes says plainly, "This passage is quoted from Isaiah 40:13," while Gill says the apostle "either cites or alludes to" it. The expected answer is no one. Barnes explains that asking it as a question is "a strong mode of denying that anyone has ever known the mind of the Lord." Paul uses it to explain why the spiritual person is "judged of no man" (1 Corinthians 2:15). If no one can instruct the Lord, then no one without His Spirit can sit in judgment on what His Spirit teaches.

Paul answers at once: "But we have the mind of Christ" (1 Corinthians 2:16). No one can teach God, yet believers have been given Christ's mind by the Spirit (1 Corinthians 2:12). Then Paul turns the point on his readers: "I, brethren, could not speak unto you as unto spiritual, but as unto carnal, even as unto babes in Christ" (1 Corinthians 3:1). Having the mind of Christ did not mean the Corinthians had grown into it.

Isaiah 40:13 in the KJV reads, "Who hath directed the Spirit of the LORD, or being his counsellor hath taught him?" It comes from Isaiah's great chapter on God's greatness, where the next verse asks, "With whom took he counsel, and who instructed him?" (Isaiah 40:14). Paul's Greek uses nous, mind or understanding, for both "the mind of the Lord" and "the mind of Christ." The verb for "instruct" is symbibazō, to teach. Paul asks the same question again in Romans 11:34.

This question puts us in our place before God. No one has ever given God advice He needed. Isaiah asked it while looking at a God who measures the waters and the heavens (Isaiah 40:12-14), and Paul asks it while speaking of God's "hidden wisdom," which, had the princes of this world known it, "they would not have crucified the Lord of glory" (1 Corinthians 2:7-8).

Yet the same verse that humbles also gives a gift: "we have the mind of Christ." Barnes explains that the Christian has the mind of Christ, "who had the mind of God." Gill notes that Paul sets "the mind of Christ" beside "the mind of the Lord," which he says "proves that Christ is the Lord, or Jehovah, and so truly and properly God." Gill adds that this is to be understood "not only of the apostles and ministers of the Gospel, but of all true believers," though chiefly of the apostles. Matthew Henry calls it "the great privilege of Christians, that they have the mind of Christ revealed to them by his Spirit."