Bring Zenas the lawyer and Apollos on their journey diligently, that nothing be wanting unto them. - Titus 3:13
Based on the passage above, is it evident whether Zenas the lawyer was a part of the early Christian church?
The scriptures do not mention Zenas anywhere else except this passage and there is no explicit assertion that he is a believer so we are dependent on inference and thus must maintain some uncertainty. However, the evidence is very strong that he was a believer in that he seems to be travelling with Apollos on business of interest to the assembly and Paul is instructing the saints to be of service to them and provide for them in their journey.
Albert Barnes in "Barnes' Notes" notes:
Bring Zenas the lawyer - - This person is not elsewhere mentioned in the New Testament, and nothing more is known of him. He belonged doubtless to that class of persons so often mentioned in the New Testament as lawyers; that is, who were regarded as qualified to expound the Jewish laws; see the notes at Mat_22:35. It does not mean that he practiced law, in the modern sense of that phrase. He had doubtless been converted to the Christian faith, and it is not improbable that there were Jews at Nicopolis, and that Paul supposed he might be particularly useful among them. And Apollos - Notes, Act_18:24. He was also well-skilled in the laws of Moses, being “mighty in the Scriptures” Act_18:24, and he and Zenas appear to have been traveling together. It would seem that they had been already on a journey, probably in preaching the gospel, and Paul supposed that they would be in Crete, and that Titus could aid them.
That Zenas and Apollos are travelling together on a trip together that is of concern to Paul and the saints suggests that they were being called in as experts in order to help resolve a dispute. Zenas would bring his knowledge of Jewish law and Apollos would bring his knowledge of the Christian faith to bear on whatever question was at hand. It is possible that the dispute was within the assembly or with the unbelieving Jews. It is less likely that it was to be discussed in a Roman court but might have been the case:
Act_23:29 Whom I perceived to be accused of questions of their law, but to have nothing laid to his charge worthy of death or of bonds. ... Act_25:19 But had certain questions against him of their own superstition, and of one Jesus, which was dead, whom Paul affirmed to be alive. Act_25:20 And because I doubted of such manner of questions, I asked him whether he would go to Jerusalem, and there be judged of these matters. ... Act_26:3 Especially because I know thee to be expert in all customs and questions which are among the Jews: wherefore I beseech thee to hear me patiently.