The eunuch had to "understand" the gospel clearly "before" the preacher would baptize him.
... Understandest thou what thou readest? (Ac.8:30)
A message always comes before any conversion.
(Jn. 6:44,45) (Acts 8:5,12,13,26-40)
Believe + Baptism = Salvation - (Mark 16:16) (1Peter 3:20,21)