This is true...at least according to the writings of Paul himself.
Once again, this is where genre and intent comes into play. There's all sorts of bias and reason to doubt parts of Paul's letters that refer to historical events. However, these support, rather than cast doubt, on the reality of the historical Jesus. Paul was a contemporary, but he wasn't a follower of Jesus. His letters indicate his attempts to legitimize his status as a disciple, comparable to Peter and other leaders of the early church. The fact that he is concerned with establishing his authority as comparable to those who were followers of Jesus before biases his account in particular ways. If Jesus was a myth, and Peter or James didn't know him as a historical individual anymore than anyone else, then Paul's "revelational" knowledge of Jesus would be no less legitimate than Peter's or James.
His letters are written to believers. He's not trying to convince anybody that the historical Jesus existed. In fact, he tends to ignore the historical Jesus (whom he almost certainly didn't know), entirely. This could be because the historical Jesus didn't exist, but even using only Paul, this explanation is very poor and a much better one is that while he didn't know Jesus while he was alive, others did. After all, he refers to Jesus' brother. One specific literal brother, identified by his kinship relationship to Jesus. There's no reason to believe that Paul was lying, or that he lacked the ability to realize James' kinship was made-up, or any other reason to doubt that Paul actually knew a relative of the living Jesus. This is only strengthened by the one unquestioned non-christian early reference to Jesus: Josephus' reference to the same brother. So even if one discounts the majority of specialists who argue that the so-called testimonium flavianum is entirely an interpolation, rather than an edited version of an original which did refer to Jesus, there is still this second reference to James Jesus' brother.