For: translates the same Greek word as since in verse 21. It is difficult to see what the purpose of this word is, and Good News Bible, Revised English Bible, and other translations omit it.

Demand and seek translate Greek verbs that are similar in meaning. They are probably used for variety. Revised Standard Version‘s demand is probably too strong. The meaning of seek is “to gain” or “expect to get.” “Expect” would be closer to the meaning here, or some phrase like “want someone to give (or, show) them.”

Signs: especially in John’s writings, the Greek word for “sign” sometimes means a miracle that does not only astonish people, but shows the meaning of what God is doing. Good News Bible‘s “miracles” is a better translation in this verse. The phrase “want miracles” can also be rendered as “want to see miracles.” Other ways to translate signs are “amazing things” or “powerful acts that amaze people.”

Good News Bible‘s “for proof” is not in the Greek. Other translations that we consulted omit it (except French common language version [Bible en français courant]). In favor of adding these words, it may be said that there is evidence in the New Testament of people asking for, or being given, miracles as signs that God was at work in Jesus (see, for example, Matt 12.38-42; Luke 7.20-21; 11.29-32; John 14.11; 1 Cor 2.4). If the words “for proof” are added, it may be necessary in some languages to explain what is being proved. This may be done by adding a few words, such as “proof that God is at work”; this is implied in verse 21. In any case, the idea of logical or mathematical proof should be avoided. What Paul means here is powerful convincing evidence; the Jews whom Paul mentions would have accepted miracles as such evidence. Paul is probably thinking, in fact, of miracles like the ten plagues in the Old Testament, which convinced the king of Egypt that he should set the Israelites free. The evidence seems to show, then, that the phrase “for proof” may be used if it is in the sense that “God is at work.”

In verses 22-24 two different Greek words are used for “non-Jews,” namely Greeks and Gentiles. It is most unlikely that Paul intended to refer to two separate groups of people. Greeks was most often used to mean, not just the Greek nation, but non-Jewish pagans in general. The translation of Jews and Greeks in verse 22, as well as Gentiles in verse 23, should avoid suggesting that Paul means every individual Jew and Gentile. He has experienced that many Jews ask for signs, and many Greeks seek wisdom, and based on that experience he has made a general statement about the two groups. In certain parts of the world it will be necessary to take care that the terms used for “Jews” and “Gentiles” are not considered insulting. “Gentiles” means generally “non-Jews.” So any reference to particular groups, such as Greeks, Arabs, or Christians, should be avoided. “Gentiles,” though, is not a common-language word; the Good News Bible word list explains it as “a person who is not a Jew.”

An alternative translation model for this verse is:
• For Jews want to see powerful and amazing acts (or, deeds) to show that God is at work; non-Jews expect to get great understanding.

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .