Acts 8:37 was in the bible from the start but was removed to make way for infant baptism when Constantine usurped Christianity and made it a state religion. By law everyone was a Christian and thus you were born into Christianity, hence the necessity for infant baptism. To support my position I need to prove Acts 8:37 existed before Constantine. Fortunately there are unbiased external references proving Acts 8:37 existed and predated the 3rd and 4th century manuscripts which are missing this verse.
Irenaeus (180 AD): [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)
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)
In addition to Acts 8:37 being referenced by the Latin author Cyprian, it was alluded to by Pontius the Deacon who wrote
“The Life and Passion of Cyprian, Bishop and Martyr,” who stated: “For although in the Acts of the Apostles the eunuch is described as at once baptized by Philip, because he believed with his whole heart..”
Cyprian’s writings were composed around A.D. 250, and his citations of Scripture is from the Old Latin that had been translated approximately a century earlier, thus presenting evidence for the verse a few decades earlier than Irenaeus’ writings. It is thus vouched for from both Greek and Latin sources extending back to the mid-second century.
— Heath Henning
Acts 8:37 is found in manuscripts of the New Testament that date from the 500ADs called the "Textus Receptus" The manuscripts were brought to Europe in the 1500s and used in translations like the King James Bible. The modern translations are translated from 3rd & 4th century manuscripts and you can view them in the British museum. So logically Acts 8:37 was not removed because it wasn't there in the 4th century manuscripts.. it was ADDED. There are a few Christians who believe the "Textus Receptus" is an uncorrupted manuscript and the earlier ones were altered.
— Paul Robinson on Windsor On Fire
Acts 8:37 is very clear. The Eunuch asks what would stop him from being baptized. Philip responds that you must believe with all your heart. Can a baby believe with all it's heart that Jesus is the Son of God? No! Therefore a baby cannot be baptized.
36 As they went along the road they came to some water; and the eunuch said, “Look! Water! What prevents me from being baptized?” 37 And Philip said, “If you believe with all your heart, you may.” And he answered and said, “I believe that Jesus Christ is the Son of God.”
— biblehub.com: Acts 8 NASB
Faith is thus seen to be a necessary predisposition in the adult, for the reception of baptism. They must answer for themselves, but infants are baptized in the faith of the Church. Their sponsors, who receive them from the font, answer for them. And as the defilement was not personal, but that of others, so are they purified by the faith of others.
— Catholic Answers: Haydock Bible Commentary