Did the Holy Spirit reveal the plan of salvation to the eunuch without a human instrument?
No. The Holy Spirit was evident, but He did not choose to reveal the plan of salvation to the Ethiopian eunuch without a human instrument. The Spirit saw fit to use Philip to explain the Scriptures, and that pattern continues for those who explain Scripture to seeking people.
Les explains that the Holy Spirit is evident in the account, but He did not choose to show the Ethiopian the plan of salvation without using the human instrument. The eunuch was reading Isaiah and could not understand it, so the Holy Spirit saw fit to use Philip to explain the Scriptures. The eunuch even asked how he could understand unless some man should guide him, and he invited Philip to come up and sit with him. Les says this is where believers come in today: we have to be ready to explain the Scriptures to someone who is interested and seeking. Faith comes by hearing, and hearing by the Word of God.