Summary: When we experience new life in Jesus Christ we might think a change in our external appearance, position, or circumstances will make us more useful to the Master. But you might just miss out on one of the greatest opportunities available to you.
Last time we talked about how some in Corinth, especially women who were just experiencing new freedoms in the body of Christ, thought it best to divorce their pagan husbands, or remain celibate in marriage in order to be better Christians. In the context of an overly-sexualized society, and of a coming persecution—it seemed to make sense to them.
But the Apostle tells them not to panic. If God calls you to be single, then be single—otherwise get married, work hard at the relationship, don't withhold yourself sexually from your spouse, and if you do get divorced, don't just rush off and marry someone else but get back with your partner unless there are good reasons for it—and there are sometimes.
In the last portion of the chapter he brings in more examples of how we might feel a radical change in our circumstances might help us be better servants of Christ. But in this section he again says "Don't Panic." There is no need to make radical changes. The real need is to reflect Christ wherever you are and let the internal changes He is making shine out.
17 – 24
Don't be worried about what you could be for God. Be concerned about what you already are!
I don't normally do this, but I love how the Message paraphrase puts verses 17-24:
1 Corinthians 7:17-24 (quickview)  And don't be wishing you were someplace else or with someone else. Where you are right now is God's place for you. Live and obey and love and believe right there. God, not your marital status, defines your life. Don't think I'm being harder on you than on the others. I give this same counsel in all the churches.
18 Were you Jewish at the time God called you? Don't try to remove the evidence. Were you non-Jewish at the time of your call? Don't become a Jew. 19 Being Jewish isn't the point. The really important thing is obeying God's call, following his commands.
20 Stay where you were when God called your name. 21 Were you a slave? Slavery is no roadblock to obeying and believing. I don't mean you're stuck and can't leave. If you have a chance at freedom, go ahead and take it. 22 I'm simply trying to point out that under your new Master you're going to experience a marvelous freedom you would never have dreamed of. On the other hand, if you were free when Christ called you, you'll experience a delightful "enslavement to God" you would never have dreamed of.
23 All of you, slave and free both, were once held hostage in a sinful society. Then a huge sum was paid out for your ransom. So please don't, out of old habit, slip back into being or doing what everyone else tells you. 24 Friends, stay where you were called to be. God is there. Hold the high ground with him at your side.
I know our tendency can be once we become a Christian to think that we need to quit our jobs and go to Africa or somewhere on the mission field and that's how we can be the best Christian. You might be called to drop everything and go to Africa, but make very sure that is what's happening and you are not simply responding to what you think would make you more holy and useful. Part of Paul's message is that your mission field is right before you, among your friends, family, co-workers, neighbors, and those you meet on the street doing your everyday stuff. It's the outward reflection of inward change that matters, not changing outward position or circumstances.