The Ethiopian was a God-fearing person, not a proselyte.
In the book of Acts we encounter God-fearing people
and proselytes.
Proselyte (prosѐlutos, from the verb “proserchomai” means to join, to come to) is an indication for heathens who have embraced the Jewish faith and became a Jew through circumcision. They counted as full-fledged Jews. Think of the Jewish mission by Pharisees (Matthew 23:15). One of the seven, a colleague of Stephen, was Nicolaus, a proselyte from Antioch (Acts 6:5). The whole group of seven has Greek names, and one of them is Philip. In Acts 2:11 we read about Jews and proselytes,
also called Jew-fellows: non-Jews who professed the Jewish religion and counted as full-fledged Jews through circumcision.
Proselytes can be distinguished from God-fearing people
(“sebomenoi ton theon”), they are non-Jewish sympathizers of the Jewish faith. They supported for example, the building of a synagogue or were interested listeners at the services and observed several Jewish regulations. They were viewed by the Jews as uncircumcised heathens. Such a God-fearing man with sympathy for the Jewish faith is for example, Cornelius (Acts 10:2, Acts 10:22). Peter begins his preaching with Cornelius: in every nation anyone who fears him and does what is right is acceptable to him
(Acts 10:35). Compare for the God-fearing also Lydia (Acts 16:14), quite a few Greeks in the synagogue of Thessalonica (Acts 17:4), and people from Athens (Acts 17:17) and Corinth (Acts 18:7). In the synagogue of Antioch (Pisidia), Paul addresses his audience as men of Israel and you who fear God
(“phoboumenoi ton theon”) (Acts 13:16, Acts 13:26). Many Jews and devout converts to Judaism followed Paul and Barnabas
(Acts 13:43). It is remarkable that the proselytes are called devout (i.e., God-fearing) here!
Proselytes are people who have joined the Jews by letting themselves be circumcised. This is obviously different when compared to God-fearing people who show interest. As the Ethiopian is an emasculated person, he could not belong to the Jewish nation as proselyte (compare Deuteronomy 23:1). (People have tried in the exegesis to make this Ethiopian into a proselyte, or someone who would have had Jewish ancestors. Behind this incorrect notion lies the thought that it is especially Peter who has the scoop of baptizing the first true heathen). Luke writes his books to the God-fearing Theophilus (Luke 1:3–4), who, just as Cornelius, has a high function as Roman officer. He wants to make clear to Theophilus that the God of Israel has taken a decisive step ahead in his Son, Jesus, by whom the promises to Israel are being fulfilled. Also in connection with that central message for the most excellent Theophilus, the encounter of Philip with the Ethiopian plays such an important role (as will the later encounter of Peter with Cornelius). He wants to bring the God-fearing people to Christ! Several times it becomes clear in the book of Acts that the Christian mission work is prepared by the Jewish mission. Proselytes and God-fearing people form a springboard to mission among the heathens.
27 And he rose and went. And there was an Ethiopian, a eunuch, a court official of Candace, queen of the Ethiopians, who was in charge of all her treasure. He had come to Jerusalem to worship