Our sermon passage, known by the scholarly term pericope adulterae (PA), is one of two large sections of Scripture under attack by the Devil's minions, such as Dr. James White and Dr. Bart Ehrman. (The other section is the last 12 verses of Mark 16.) You know the rant: "It's not found in the oldest most reliable manuscripts", by which they mean an absence from the twin golden calves of Alexandrinus and Sinaiticus. Never mind that their witness agreeth not, nor that either are the oldest.
There is ample internal and external support for the passage to absolutely be the God-breathed Word of the Lord. Internally, the context begins in John 7:2 with Jesus' expected arrival at the feast of Tabernacles. In the middle of the feast, He begins publicly teaching (7:14), which causes a stir among the people (7:31), catching the Pharisees' attention who send officers to take Jesus (7:32). Many accepted Him as the Messiah, preventing any man from laying hands on Jesus (7:44). When the officers return, the chief priests and Pharisees complain about their failure to bring him (7:45). The chapter ends with everyone going to their own house. If the PA were not inserted, the next section would begin at John 8:12 with Jesus telling the Pharisees, I am the light of the world – leaving one to wonder: (a) when did Jesus get an audience with the Pharisees? (b) when did we transition from a private to a public scene? It makes zero sense.
There are several good resources which nicely summarize the external evidence for the PA; what follows is a summary of ecclesia.org's defense . (Although a bit cryptic, it is a citation of manuscripts containing the PA, as well as reliable church fathers who cited the passage.) "The [PA verses are] found in over 900 manuscripts. ° Fuller cites [Dean John] Burgon as stating that of 73 copies of John's Gospel in the British Museum, 61 contain John 7:53-8:11 as found in this passage. Burgon indicates that this proportioning would be typical for any collection of manuscript copies of John. He also cites a further 60 copies, from three distinct lines of ancestry, which agree with this passage."
"Jerome (385 AD), included it in the Vulgate after surveying older Greek copies, stating it was found 'in many copies both Greek and Latin', before 415 AD. The Ethiopic (5th century), Palestinian Syriac (5th Century), Georgian (5/6th century), some copies of the Armenian (4/5th century), Slavonic, Arabic and Persian versions Ambrose (374 AD), Augustine (396), Chrysologus (433), Faustus (400), Gelasius (492), Pacian (370), Rufinus (400), Sedulius (434), Victorius (457), Vigilius (484) and others [contain it]. The Lectionary practice of the Eastern Church, from the 2nd century, include the PA."
"Burgon states that the dislocation of John 7:53-8:11 is attributable to four cursives, 13, 69, 124, 346, all evidently from one ancient and corrupt copy. Ruckman cites [the following] in favor of the passage: the Didache (3rd century document of Apostolic Teachings), Apostolic Constitutions (4th century) and Eusebius (324 AD) citing Papias (150 AD) as recognizing the passage. The Montanists (2nd century) were also aware of the passage. Ruckman also cites besides D, uncials M, S and Gamma from the 5th, 8th and 9th centuries in favor of this passage."
Eminent textual scholar, Edward F. Hills, provides this interesting anecdote: "Although the Greek Fathers were silent about the pericope de adultera, the Church was not silent. This is shown by the fact that John 8:3-11 was chosen as the lesson to be read publicly each year on St. Pelagia's day, October 8. Burgon points out the significance of this historical circumstance. 'The great Eastern Church speaks out on this subject in a voice of thunder. In all her Patriarchates, as far back as the written records of her practice reach, —and they reach back to the time of those very Fathers whose silence was felt to be embarrassing,—the Eastern Church has selected nine out of these twelve verses to be the special lesson for October 8." 
There is ample support for the PA. Who else but Jesus could drive away her accusers and say, neither do I condemn thee? This wasn't the first nor the last time our Savior shocked the world by whom He saved.
 http://www.ecclesia.org/truth/manuscript_evidence.html ; scroll to section on John 8.
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