"Was a man already circumcised when he was called? He should not become uncircumcised. Was a man uncircumcised when he was called? He should not be circumcised. Circumcision is nothing and uncircumcision is nothing. Keeping God's commands is what counts." NIV translation
Paul is using the Jewish tradition of circumcision to make his point. That point once again is that we are saved by the grace of God and not by our own actions. Once we are saved, we are to follow Jesus as His disciple.
"Each one should remain in the situation which he was in when God called him. Were you a slave when you were called? Don't let it trouble you - although if you can gain your freedom, do so. For he who was a slave when he was called by the Lord is the Lord's freedman; similarly, he who was a free man when he was called is Christ's slave." NIV translation
Just as you do not try to go back and change a circumcision, Paul tells us that we are to accept where we are and to follow the Lord as He directs. Paul is talking about being a bondslave when he speaks of being "Christ's slave". This is how he often referred to himself and it speaks of someone who was set free by their master but willingly stays to serve that master.
"You were bought at a price; do not become slaves of men. Brothers, each man, as responsible to God, should remain in the situation God called him to." NIV translation
Paul is talking about becoming slaves to the rules of men instead of listening to God and what he has for your life. He reminds us that we answer to God and not men and so we must listen to Him.
"Now about virgins: I have no command from the Lord, but I give a judgment as one who by the Lord's mercy is trustworthy. Because of the present crisis, I think that it is good for you to remain as you are. Are you married? Do not seek a divorce. Are you unmarried? Do not look for a wife. But if you do marry, you have not sinned; and if a virgin marries, she has not sinned. But those who marry will face many troubles in this life, and I want to spare you this." NIV translation
The "present crisis" that Paul refers to is the fact that sex had become a religion in this city. Paul is reminding the Christians of the fact that, in God's eyes, sex is associated with marriage and so it is not to be taken lightly. He tells them that it is good to stay a virgin but it is also good to be married. It warns them that it will not be easy to be married in this city with the rampant abuse of the marriage relationship. This passage speaks to events that are going on in many countries today as the marriage relationship (as defined by governments) is going against the Word of God. With the fact that governments will give people a certificate to say that men can marry (have sex) with men and women can marry (have sex) with women, that piece of paper stands against the Word of God. The advice that Paul gives the church in Corinth can certainly be applied to our world today. We are free to get that piece of paper but we are also free to say "No, I do not want any part of your paper".