Acts 8:36 ...and the eunuch said,
See, here is water; what doth hinder me to be baptized?
Acts 8:37 And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
Acts 8:38 And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
Mark 16:16 He that believeth and is baptized shall be saved;
but he that believeth not shall be damned.
Acts 8:35 Phillip preached the gospel of Jesus Christ to him, and by the same the eunuch somehow understood he was to be baptized and pointed out water to Phillip (as if Phillip informed him of such a baptism)
The eunuch said,
See, here is water; what doth hinder me to be baptized? Acts 8:36
The only thing that should hinder him would be unbelief, since Phillip said,
Acts 8:37 And Philip said, If thou believest with all thine heart, thou mayest.
Makes sense since Jesus even places belief in the gospel before baptism
Hearts can still be purified by faith and have the Holy Ghost before baptism, even John had the Holy Ghost from his mothers womb (Luke 1:15) but still had need to be baptized (Mat 3:14) and Jesus also baptized in water (or rather his own disciples did so) John 4:1-2
The same with the Gentiles, which has already believed, and had the Holy Ghost before being baptized also
Of these Peter says,
Acts 10:47-48 Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we? And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days.