1 Corinthians 7:22 (ESV) … “For he who was called in the Lord as a bondservant is a freedman of the Lord. Likewise he who was free when called is a bondservant of Christ.”
In most languages the word For can be omitted. It is in the Greek, but Paul is not speaking here about a consequence or result of verse 21. Rather he is introducing a series of further points that are related to the themes of Christian freedom and being a servant (or slave) of Christ. A slave is a person who is the property of someone else. He or she has no rights and must show complete obedience and loyalty to the owner. A possible alternative translation of this word in cultures where slaves are unknown is “a person who is the property of (or, owned by) another.”
In the Lord: it is more common for New Testament writers to speak of God, rather than of Christ, as calling people to faith. “In the Lord” and “in Christ” are favorite expressions of Paul to express the believer’s unity with Christ. The immediate context of this verse suggests that the meaning is “called to belong to the Christian fellowship” or “called to be one with Christ” rather than “called by the Lord,” that is, “by Christ.”
The word that is translated freedman is not used elsewhere in the Greek Bible but is fairly common in Greek secular literature. It refers to a former slave who has been set free, not someone who has always been free like Paul himself (see Acts 22:28). This raises the question, by whom has he been set free? The use of the Lord here, and the mention of Christ later in the verse, strongly suggests that the meaning is “a slave whom Christ has freed.” Similarly, in the second half of the verse, “someone who serves Christ” implies “a slave who belongs to Christ.”
An alternative translation model for this verse is: For a slave whom God has called to be one with Christ is a slave whom Christ has freed; in the same way a free man whom God has called to be one with Christ serves him.
 Ellingworth, P., Hatton, H., & Ellingworth, P. (1995). A handbook on Paul’s first letter to the Corinthians (pp. 161–162). New York: United Bible Societies.