"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."
Many of the newer translations following the Egyptian manuscripts of the 3rd through 9th centuries do not have this verse however arguments of doctrinal issues from before their time contain it. If it were not around prior to the 3rd century then how can the NIV scholar explain:
Irenaeus (180 AD): But again: Whom did Philip preach to the eunuch of the queen of the Ethiopians, returning from Jerusalem, and reading Esaias the prophet, when he and this man were alone together? Was it not He of whom the prophet spoke: "He was led as a sheep to the slaughter, and as a lamb dumb before the shearer, so He opened not the month? ""But who shall declare His nativity? for His life shall be taken away from the earth."194 [Philip declared] that this was Jesus, and that the Scripture was fulfilled in Him; as did also the believing eunuch himself: and, immediately requesting to be baptized, he said, "I believe Jesus Christ to be the Son of God."(Against Heresies, 3.12.8)(http://www.tertullian.org/fathers2/ANF-01/anf01-60.htm#P7297_1937859)
Cyprian (250 AD): In the Acts of the Apostles: "Lo, here is water; what is there which hinders me from being baptized? Then said Philip, If you believe with all your heart, you may." (The Treatises of Cyprian, Treatise 12, Book 3.43) http://www.newadvent.org/fathers/050712c.htm
"The eunuch believed on Christ, and said when they came unto a certain water, See water, who doth hinder me to be baptized? Philip said to him, Dost thou believe on Jesus Christ? He answered, I believe that Jesus Christ is the Son of God. Immediately he went down with him into the water." Augustine (354-430 AD), Sermon 49
Granted these men seem to already have been looking at baptism as a means of salvation or forgiveness, interpreting their own view into the text by ignoring The apostle Peter's statement that the unbaptized gentiles received the Spirit of God the same way the Jews of acts 2 had received him that being by faith. ( Acts 2 and 10 compared)
The point here is not what they thought it meant but that these scriptures were already understood to be inspired and included by God as Genuine statements made by both Philip and the eunuch.
Dear Saint, after being convinced through both reading the Bible and having Phillip expound upon the scriptures that Jesus was and is truly the Christ, the eunuch must have been told, or understood that believers were to follow the Lord in baptism, quite possibly Phillip even told him that when they arrived at some church that they he might be baptized but the eunuch saw water and asked a simple question to which the New versions imposing as Bibles give no answer.
CONTEXT: Acts 8: 34 And the eunuch answered Philip, and said, I pray thee, of whom speaketh the prophet this? of himself, or of some other man?
35 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.
36 And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized?
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.
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.
39 And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on his way rejoicing.