The phrase "a certain Jew" appears in the New Testament, specifically in the Book of Acts, and is used to introduce individuals whose specific names may not be immediately mentioned or who play a particular role in the narrative. This phrase highlights the Jewish identity of the person in question, often setting the stage for events that involve interactions between Jewish customs and the burgeoning Christian faith.

Acts 18:24-26 :
"Meanwhile a Jew named Apollos, a native of Alexandria, came to Ephesus. He was a learned man, well-versed in the Scriptures. He had been instructed in the way of the Lord and was fervent in spirit. He spoke and taught accurately about Jesus, though he knew only the baptism of John. He began to speak boldly in the synagogue. But when Priscilla and Aquila heard him, they took him in and explained to him the way of God more accurately."

In this passage, Apollos is introduced as "a Jew named Apollos," emphasizing his Jewish heritage and his role as a learned teacher who initially had an incomplete understanding of the Christian message. His encounter with Priscilla and Aquila, who were also Jewish Christians, illustrates the dynamic interaction between Jewish tradition and the new teachings about Jesus.

Acts 19:13-14 :
"Now there were some itinerant Jewish exorcists who tried to invoke the name of the Lord Jesus over those with evil spirits. They would say, 'I bind you by Jesus, whom Paul proclaims.' Seven sons of Sceva, a Jewish chief priest, were doing this."

Here, the phrase "a certain Jew" is not explicitly used, but the narrative introduces Jewish exorcists, specifically the seven sons of Sceva, who attempt to use the name of Jesus in their exorcisms. This account highlights the intersection of Jewish religious practices with the emerging Christian faith and the power associated with the name of Jesus.

Acts 9:23-25 :
"After many days had passed, the Jews conspired to kill him, but Saul learned of their plot. Day and night they watched the city gates in order to kill him. One night, however, his disciples took him and lowered him in a basket through a window in the wall."

In this context, "the Jews" refers to certain Jewish individuals or groups who opposed Saul (later Paul) after his conversion to Christianity. While not directly using the phrase "a certain Jew," the passage underscores the tension between Saul's former Jewish identity and his new role as a proponent of the Christian faith.

The use of "a certain Jew" or references to Jewish individuals in the Book of Acts serves to illustrate the complex and often contentious relationship between early Christians and the Jewish community. These narratives provide insight into the challenges faced by early Christians as they navigated their Jewish heritage while embracing a new faith centered on Jesus Christ.