It's missing in some of the more traditional versions like the KJV and the NIV, but in other versions, like the ESV and amplified, they are in italics or a footnote. Is it because it conflicts with the holy trinity?
AMP - 37 And Philip said, If you believe with all your heart [if you have a conviction, full of joyful trust, that Jesus is the Messiah and accept Him as the Author of your salvation in the kingdom of God, giving Him your obedience, then] you may. And he replied, I do believe that Jesus Christ is the Son of God.
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I believe it enforces the teaching of the Trinity
I possess a KJV Bible and it is Acts 8:37, so it is wrong to say that this scripture is not in the KJV