I Corinthians 2:15

But he that is spiritual judgeth all things, yet he himself is judged of no man (KJV)

Then: Paul stops and treads water just a bit. He wants to be absolutely clear that he's covered the subject of the Holy Spirit residing within and how that impacts believers. Each spiritual person, that is each one who has the spirit, is able to make good decisions about problems, situations, people, personal actions, or any aspect of life here on earth. What he does is accountable to Christ, not to anyone on earth - even ultimately, fellow believers.

Now: A secular world confuses the difference between right and wrong. It often teaches everything is gray and that there is no way to judge or know. On the contrary, Paul shows us that believers can make decisions according to the values of God. Collins (p. 136-137) uses the term "testing," which often brings to mind "testing the spirits." That's a different dimension of judging. More than deciding what actions to take, it goes to the level of deciding who is being followed in making a decision. Who is advocating a particular choice? God? Corruptible mankind? Satanic forces? The followers of Christ choose rightly and cannot be judged by anyone but the Lord.

Extra Source: First Corinthians by Raymond F. Collins, ed. by Daniel J. Harrington (Liturgical Press, Collegeville MN, c1999) on line at http://books.google.com/books?id=c4VXq9fKoRwC&dq=Collins+Harrington+Corinthians&printsec=frontcover&source=in&hl=en&ei=uogjS7rkLJDflAfk2uX_CQ&sa=X&oi=book_result&ct=result&resnum=11&ved=0CCkQ6AEwCg#v=onepage&q=&f=false