The Christian community in Jerusalem was particularly suspicious of Paul primarily because "Paul had previously persecuted Christians." Before his conversion, Paul, who was known as Saul, actively sought to imprison and persecute followers of Jesus. His transformation into a Christian leader raised concerns among the early believers, who remembered his previous actions against them. This history made it difficult for some in the community to fully trust him, even after he declared his faith in Christ.
Other factors, such as his "strange ideas" about the inclusion of Gentiles and the nature of the law, may have contributed to their wariness as well, but his past as a persecutor was the most significant reason for their initial suspicion.