an eunuch of great authority - Eunuchs were generally employed for confidential offices in the East, and to some extent are still.
Candace - the family name of the queens of Upper Egypt, like Pharaoh, Caesar, etc. (as appears from classic authors).
had come to Jerusalem to worship - that is, to keep the recent feast of Pentecost, as a Gentile proselyte to the Jewish faith. (See Isa 56:3-8, and Joh 12:20).