The detail noted by the writer of Acts, that he was also a eunuch, compels us to suppose that he was not a full proselyte, a "proselyte of righteousness," but only a "proselyte of the gate." For, according to the Law of Moses (Deut 23:1), a eunuch could not be received into the assembly of Jehovah.
The event of the Ethiopian man's conversion to faith was especially important because, in this case, not merely an unclean Jew, and not even a Samaritan, but a Gentile by birth and blood, though a Jew by faith, was counted worthy to be received as a member of the Church of Christ.
Furthermore, by the place of his residence this Ethiopian man was also a Gentile. Consequently, through his conversion to Christ, the Church spread far beyond the boundaries of orthodox Judea and even directly into a Gentile country.
For this reason the Ethiopian man's conversion to faith in Christ is accomplished by a special and extraordinary prompting and command. An angel of the Lord visibly appears to Philip while he still remained in Samaria, and most precisely determines the place toward which he must direct his preaching path.
Even this was not enough. When Philip came to the place indicated by the angel and saw there the Gentile dignitary sitting in his chariot and occupied with the reading of the word of God, a special prompting of the Spirit of God came to Philip. He was to approach with the Gospel of Christ precisely to this man.
The opinion of some interpreters that he was a Jew born in Ethiopia rests on the fact that he was reading the prophet Isaiah. This would be sound if one supposed that he was reading the prophetic book in the Hebrew text, and not in the Greek translation.
A proselyte is a person who has adopted a new confession of faith, in this case the Jewish one.
In the East, not only castrated men in the strict sense were called eunuchs, but also all persons holding the highest state offices. Yet that the word eunuch is used here in the strict sense is indicated by the fact that this eunuch was a dignitary serving a queen. It is also indicated by the fact that otherwise the word eunuch would be superfluous here, since this man's high position and office are designated in the same place by other words: a dignitary of Candace, which was a common title in Ethiopia like Pharaoh in Egypt, queen of the Ethiopians, and keeper of all her treasure.