What the texts say. In verse 36 the eunuch asks, “See, here is water; what doth hinder me to be baptized?” In the shorter text Philip simply orders the chariot to stop and baptizes him (verse 38). The longer text adds verse 37: Philip requires faith “with all thine heart,” and the eunuch confesses that Jesus Christ is the Son of God.
The manuscript evidence. The verse is absent from the early papyri P45 and P74, from Codices Sinaiticus, Alexandrinus, Vaticanus and Ephraemi, from the Syriac and Coptic versions, and from many minuscules. Unusually, it is also missing from the majority of Byzantine Greek manuscripts, which is why the Robinson–Pierpont Byzantine edition omits it. It is found in a minority of Greek manuscripts and widely in the Latin tradition. It stands in the Textus Receptus, and so in the KJV. That it is not part of the earliest text of Acts is the near-unanimous judgment of textual scholars (Strong).
But it is early. St Irenaeus, writing about 180, says that the eunuch, asking for baptism, said: “I believe Jesus Christ to be the Son of God” (Against Heresies 3.12.8). So the confession was attached to this story by the second century, at least in some copies (Strong). It reflects how the early Church joined baptism to a spoken confession of faith.
What the Orthodox Church reads. The Patriarchal edition of 1904 prints the verse, but in small type, which its editors used for words lacking sufficient Greek support (the digital edition marks such words with brackets). Its content matches the practice the Church already shows in Irenaeus: baptism joined to a confession that Jesus Christ is the Son of God.
How translations handle it. The KJV, NKJV, Douay-Rheims and Reina-Valera 1909 print verse 37 in the text; the NKJV notes that both the critical and the Majority texts omit it. The ESV, NIV, NRSVUE and RSV Catholic Edition put it in a footnote, which is why their verse numbers jump from 36 to 38.
What it does and does not change. Nothing in the verse is foreign to the New Testament. Confessing Jesus as Lord and believing in one’s heart are joined in Romans 10:9, and Jesus is confessed as the Son of God throughout Acts and the Gospels. Whether verse 37 stands in the text or in a note, the eunuch is baptized as a believer in Christ.