I Corinthians 7:17

But as God hath distributed to every man, as the Lord hath called every one, so let him walk. And so ordain I in all churches (KJV)

Then: Paul says the same thing here as he does in verse 20 and verse 24. Serve Christ where you are and walk in the path you are already given. Henry talks about the place where one lives, the circumstances or status of the individual when called. It's sensible advice and gives a good workable guideline to the Corinthians. They already know how to provide for themselves, live in their current families, and have access to many people who do not know Christ. Their mission field is right there. The indwelling Spirit will take care of ensuring spiritual maturity as He sees fit. The believer need only concentrate on enlarging the church.

Now: N. T. Wright (p. 86) discusses God's call to the "nobodies" of everyday life. God does his work neighbor-to-neighbor by using those who have already joined the church and are able to exemplify what it means to be a follower in their designated circumstances. Paul's directions do not mean a Christian should never change status or work. Paul himself became a traveling missionary after he saw the risen Lord. The key is "as the Lord hath called." The Lord's plan may reveal work in a foreign country, use of intelligence to become a doctor, or use of talent to write great music. The believer does not initiate change, only responds, as the Spirit directs.

Extra sources: Commentary on the Whole Bible (Acts to Revelation) by Matthew Henry, online see http://www.ccel.org/ccel/henry/mhc6.iCor.viii.html