Acts 8:26-39 describes the conversion of an Ethiopian eunuch through Philip’s evangelism. The KJV includes Acts 8:37, the Ethiopian eunuch’s confession of faith:
“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.”
The NKJV, following the Textus Receptus, includes this verse. The NASB includes the verse in brackets. Translations such as the NIV and ESV omit the verse.
Early manuscripts such as P45 (3rd century), P74 (7th century), Sinaiticus (4th century), Vaticanus (4th century), Alexandrinus (5th century), C (5th century), L (8th century), and Ψ (9th century) omit the verse. The earliest extant manuscript to include the verse is E from the 6th century. Thus E is predated by 5 manuscripts that omit the verse. Worthy of note is that all 5 of these are Egyptian manuscripts. However, early Latin fathers such as Irenaeus and Cyprian knew of the 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)
These Church father quotations predate all of the extant manuscripts that omit the verse. In terms of the number of manuscripts, 8:37 is not found in the majority of even the later manuscripts. However, the official Greek text of the Greek Orthodox Church, the 1904 Patricarchal Text, has included the verse:
“εἶπε δὲ ὁ Φίλιππος· εἰ πιστεύεις ἐξ ὅλης τῆς καρδίας, ἔξεστιν. ἀποκριθεὶς δὲ εἶπε· πιστεύω τὸν υἱὸν τοῦ Θεοῦ εἶναι τὸν ᾿Ιησοῦν Χριστόν.” (1904 Patriarchal Text, Acts 8)
From the external evidence, it appears the verse was missing very early in the Egyptian churches. The Latin churches had the verse in their Latin Bibles and perhaps even in their Greek copies.
Cause of Addition or Omission
As is typical of longer readings in the Textus Receptus, critics of the inclusion of 8:37 propose that the verse was a pious addition. Inversely, proponents of the inclusion of 8:37 propose that the verse was omitted by scribes who disliked the message of the verse. Origen of Alexandria, Egypt in the 3rd century testified that manuscripts in Alexandria underwent corruption by way of careless or unfaithful copying. He said:
“…the differences among the manuscripts [of the Gospels] have become great, either through the negligence of some copyists or through the perverse audacity of others; they either neglect to check over what they have transcribed, or, in the process of checking, they lengthen or shorten, as they please.”
(Bruce Metzger, The Text of the New Testament: Its Transmission, Corruption, and Restoration, 3rd ed. (1991), pp. 151-152).
So the theory that the verse was omitted early is certainly viable. Let us see which theory is more likely in the context of the early church and its theology regarding baptism.
The message of the verse is that baptism prerequires the expression of faith in Jesus Christ. This is a message that is central to many Evangelical churches today. However, this message was not believed by the predominant churches of the early centuries (or even today among the mainline denominations). The common belief among the early churches was that even infants, who could not express faith, could be baptized. Here are some quotes by influential church fathers supporting infant baptism:
Origen (244 AD): “Baptism is given for the remission of sins; and according to the usage of the Church, Baptism is given even to infants. And, indeed, if there were nothing in infants that required the remission of sins and nothing in them pertinent to forgiveness, the grace of Baptism would be superfluous.” (Origen, Homily on Leviticus 8:3 — AD 244)
Cyprian (250 AD): “But in respect to the case of infants, which you say ought not to be Baptized within the second or third day after their birth, and that the law of ancient circumcision should be regarded, so that you think one who is just born should not be Baptized and sanctified within the eighth day ….And therefore, dearest brother, this was our opinion in council, that by us no one ought to be hindered from Baptism …we think is to be even more observed in respect of infants and newly-born persons.” (Cyprian, Epistle 58, To Fides  — AD 251)
Gregory Nazianzus (381 AD): “Be it so, some will say, in the case of those who ask for Baptism; what have you to say about those who are still children and conscious neither of the loss nor of grace? Are we to Baptize them too? Certainly, if any danger presses. For it is better that they should be unconsciously sanctified than that they should depart unsealed and uninitiated.” (Gregory Nazianzus, Oration on Holy Baptism, 40:28 — AD 381)
Chrysostom (388 AD): “We do Baptize infants, although they are not guilty of any [personal] sins.” (John Chrysostom, Ad Neophytos — AD 388)
Ambrose (387 AD): “Unless a man be born again of water and the Holy Spirit, he cannot enter the kingdom of God. No one is excepted: not the infant, not the one prevented by some necessity.” (Ambrose of Milan, Abraham 2,11,84 — AD 387)
Augustine (415 AD): “Likewise, whoever says that those children who depart out of this life without partaking of that Sacrament (Baptism) are alive in Christ, certainly contradicts the apostolic declaration and condemns the universal Church, in which it is the practice to lose no time and run in haste to administer Baptism to infant children, because it is believed as an indubitable truth, that otherwise they cannot be made alive in Christ.” (Augustine, Epistle 167 — AD 415)
Council of Carthage (418 AD): “Canon 2: Likewise it has been decided that whoever says that infants fresh from their mother’s wombs should not be Baptized …let him be anathema.” (Council of Carthage, AD 418)
In both the Greek East and the Latin West, infant baptism was considered normative and beneficial. In this cultural and theological context of favoring infant baptism, what “pious scribe” would add a verse that goes against the prevalent view of his church? The theory that a “pious scribe” added 8:37 is shared by Evangelicals who see historical theology only through an Evangelical bias. The fact of history demonstrates that the message of 8:37 was subversive to the dominant theologies of the early churches. In light of this, it is more likely than not that 8:37 was omitted rather than added. The fact that 8:37 remained in the Latin stream despite the Latin church’s deeply held devotion to infant baptism demonstrates the resilience of 8:37, which is best explained by the theory that 8:37 was supported by sufficient external evidence in early times.