Every day, we hear, watch and read things.  We accept some and reject others on the basis of trust.  That trust is based on our confidence in the integrity of the person or organisation, on shared values and beliefs.  Therefore it is obvious that a person who does not know God, and has not received God's Spirit, will not be able to understand, value or trust what He is saying.  They assume that the words and works of God are foolish.

By contrast, believers have God's Spirit within them.  He explains to them what the Bible means.  He guides God's people to understand what He is doing and how to work with Him.  The things of God are important to them and with His help they are able to make godly decisions about how to live and witness in a corrupt world.  Human wisdom has some value, but believers should not be controlled by those ideas: the Spirit enables them to discern what is acceptable to God and what should be rejected.

Paul's quote from Isaiah 40:13 (Who has known the mind of the Lord so as to instruct Him?) was a reminder to Israel that God knows everything.  He does not need teaching from pagan gods or worldly wisdom.  Paul then applied that to believers: 'instead of trying to teach God, let the Holy Spirit teach you … the mind of Christ is within you because you have the Spirit … listen to Him … then you can make godly decisions'.

Like the Christians in Corinth, we can let the opinions of other people control our lives, and make choices based on fear of being rejected by our friends: or we can be guided by the Spirit.  But how do we distinguish between the voice of the Spirit and our own desires?  That is why we have the Scriptures.  The Spirit and the Word always agree.  The challenge for every believer is to see the world through the lens of the Word, and allow the Spirit to guide them through the complexity of workplace and social interactions so that God's will is done.