Philip asked him if he understood what he read and he did not. Notice that the angel did not instruct the Eunuch, but Philip preached to him. God uses men to preach his message. Let's continue to see what kind of faith hearing leads us to. Belief is required.
Belief is more than just mental assent.
Just acknowledging, or agreeing, that Jesus is the Son of God is a dead faith. We must have an obedient faith. Belief must lead to action. I know that thou believest. Agrippa had heard the prophets and believed, but that belief was dead because he did not act on it.