What baptism do you believe Paul refers to in 1 Corinthians 1?
Only Jews are to be baptized?
Well first I'd recommend reading my other comments in the thread but...
Of course there were Gentiles baptized also, but let me try to explain. The ritual that was named baptism in the NT was a common practice at least as early as the intertestamental period. For my next comment, consider the next passage:
Acts 15:20-22
20 But that we write unto them, that they abstain from pollutions of idols, and from fornication, and from things strangled, and from blood. 21 For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day. 22 Then pleased it the apostles and elders with the whole church, to send chosen men of their own company to Antioch with Paul and Barnabas; namely, Judas surnamed Barsabas and Silas, chief men among the brethren:
Years after Peter preached to Cornelius, Paul had been taking the gospel to the Gentiles and eventually the Jerusalem council came to recognize Paul's unique apostleship and gave him the right hand of fellowship. Take note that Paul always went to the synagogue first everywhere he went. Why? Why wouldn't he? He desired his kinsmen in flesh be saved, but the point I'm trying to make is that 'in EVERY CITY Moses was being read in the synagogues every sabbath'.
Acts 13:42
And when the Jews were gone out of the
synagogue, the
Gentiles besought that these words might be preached to them the next sabbath.
My point being, they would have been very familiar with the Jewish ritual or custom of baptism, especially for conversion. Gentiles in every city were accepting the Jewish Messiah, so submitting to water baptism would not have been a strange thing at all.
Today, all over the world, the church is predominately Gentile. We aren't going to the synagogues to learn about God. It's also no surprise that by in large Christendom has adopted the custom or ritual of baptism as their own, but it's also one of the most debated topics in all of Christianity. Who performs it, how is it done, when is it done, what does it accomplish, and on and on it goes.
Notice in Acts 15 the instruction to Gentiles from the council to abstain from pollutions of idols. Have you had that issue lately? They were surrounded by Jews and they were not to offend them. As we know Paul goes on to write to them saying, the idol is nothing in the world and if someone bids you to a feast, ask no question for conscience sake (somebody else's conscience).