What scripture are you basing that idea on?
Acts 8, for one, the story of the Ethiopian eunch.
Philip, under the guidance of the Holy Spirit, approached the eunuch's chariot and asked him if he understand what he was reading in Isaiah.
The eunuch said no, so Philip got in the chariot and expounded the passage, pointing him to Jesus as the fulfillment.
Afterward, the eunuch asked Philip, "okay, here's some water. What's to keep me from being baptized?"
Philip answered "If you believe, you may."
The eunuch said, "I believe that Jesus Christ is the Son of God."
That was good enough for Philip.
The point is that Philip extensively explained the scriptures, and then made sure that he understood that faith precedes baptism.
When the eunuch said he believes Jesus Christ is the Son of God, he was making a pretty broad statement. He was saying that Jesus was the Messiah, the Anointed One Isaiah wrote about. He was also confessing that he believed Jesus was God in the flesh. And he was confessing that he understood the significance of Jesus' death on the cross to save sinners. And he did not understand that until taught by Philip.