Sceptical, have you ever pondered why Muhammad stayed monogamous for 25 some years? Have you heard about his first wife's cousin Waraqa Ibn Nawfal?
He was also one of the four hanifs mentioned by Ibn Ishaq in his biography of Muhammad.
Ibn Ishaq relates (as it appears in ibn Hisham's recension):
Waraqa attached himself to Christianity and studied its scriptures until he had thoroughly mastered them."
(Ibn Ishaq, Life of Muhammad, tr. Guillaume, 1967, p. 99)
This means that the scriptures were uncorrupted during his time. It is said that when Muhammad received his first visions, Khadija came to consult him. Muslims have claimed that Waraqa represents the pure, uncorrupted form of Christianity, and who deny the Christian Trinity. However, there is no evidence that Waraqa was a Unitarian Christian. In fact, historical evidence appears to contradict this. The churches in the Arabian peninsula were either in communion with the Monophysitic Church in Egypt or the Nestorian Church in Constatinople (and these two had long been in antagonistic relationships). Both branches believe that Jesus was divinely God; they differed in how the human and divine natures of Jesus interacted when He was man on this earth. The Qur'anic teaching are more in line with the Nestorian teaching.
Now, I am not saying Waraqa is the author but his influence is undeniable on the family.
Narrated 'Aisha: Sahih Bukhari volume 1, Book 1, Number 3
Khadija then accompanied him to her cousin Waraqa bin Nawfal bin Asad bin 'Abdul 'Uzza, who, during the pre-Islamic Period became a Christian and used to write the writing with Hebrew letters. He would write from the Gospel in Hebrew as much as Allah wished him to write. He was an old man and had lost his eyesight. Khadija said to Waraqa, "Listen to the story of your nephew, O my cousin!" Waraqa asked, "O my nephew! What have you seen?" Allah's Apostle described whatever he had seen. Waraqa said, "This is the same one who keeps the secrets (angel Gabriel) whom Allah had sent to Moses. I wish I were young and could live up to the time when your people would turn you out." Allah's Apostle asked, "Will they drive me out?" Waraqa replied in the affirmative and said, "Anyone (man) who came with something similar to what you have brought was treated with hostility; and if I should remain alive till the day when you will be turned out then I would support you strongly." But after a few days Waraqa died.
Narrated 'Aisha: Sahih Bukhari Volume 9, book 87, number 111
Khadija then accompanied him to (her cousin) Waraqa bin Nawfal bin Asad bin 'Abdul 'Uzza bin Qusai. Waraqa was the son of her paternal uncle, i.e., her father's brother, who during the Pre-Islamic Period became a Christian and used to write the Arabic writing and used to write of the Gospels in Arabic as much as Allah wished him to write. He was an old man and had lost his eyesight. Khadija said to him, "O my cousin! Listen to the story of your nephew." Waraqa asked, "O my nephew! What have you seen?" The Prophet described whatever he had seen.
Waraqa said, "This is the same Namus (i.e., Gabriel, the Angel who keeps the secrets) whom Allah had sent to Moses. I wish I were young and could live up to the time when your people would turn you out." Allah's Apostle asked, "Will they turn me out?" Waraqa replied in the affirmative and said: "Never did a man come with something similar to what you have brought but was treated with hostility. If I should remain alive till the day when you will be turned out then I would support you strongly." But after a few days Waraqa died and the Divine Inspiration was also paused for a while and the Prophet became so sad as we have heard that he intended several times to throw himself from the tops of high mountains and every time he went up the top of a mountain in order to throw himself down,
According to Fr. Louis Shikho , "The Nasara among pre-Islamic Arabs", more on the influence;
The Quran does talk about "nasara" in Arabic. Some authors think that the word designates CHRISTIANS, without any further details. Others, struck by the strange beliefs attributed to the "nasara" by the Quran, think that "NASARA" designates Jewish Christian sects, especially Ebionites, Gnostics, Nazarenes who - either coming ethnically from a Jewish extraction or "converted" to such sects (like Waraqah bin Nawfal according to Ibn Hisham).
In fact, the Quran seems to describe the "nasara" as:
1- holding both the Torah of Moses (Musa) and the Injil of Isa (Gospel of Jesus)
2- believing only in "the Injil", the one and only Gospel, apparently the "Evangelium juxta Hebraeos"
3- of Hebrew or Aramaic Gospel attributed to Matthew,
4- by the Ebionites.
5- forbidding to eat pork; perhaps keeping also the sabbath, but probably circumcision.
6- denying the divinity of Christ (other times, both the Quran and the nasara seem to affirm it).
The Ebionites had both streams.
OTHER scholars say that those "nasara" were NON-Chalcedonian Christians like Jacobites, nestorians, monophysists - respectively denying the humanity of Christ, the two natures in Him.
The authors who think that the "NASARA" of the Quran designates more or less and/or pseudo Christian sects especially of Jewish origin and affiliation, believe that Christians children of the Church , in our Christian Orthodoxy, are called "RUUM" ÉϘ (Byzantines) by the Quran. It is risky to affirm categorically that "Islam" underwent the influence of this or that sect, however it seems likely that:
1- the Quran denied the divinity of Christ due to ebionite and arian influence.
2- the Quran confused between divinity and humanity of Christ due to the nestorian influence.
3- the Quran denied the crucifixion of Christ due to the gnostic docetes who claimed that Jesus had only the appearance of a body, cf. the apocryphal (or pseudepigraphic) Gospel of Peter.
4- the Quran criticized "Christians" for believing in three gods: Allah, Isa and Mary - perhaps due to the existence of a "marianistic" heresy.
Basic references about the various sects and heresies in the early Church : St. Iraeneus : "Adversus Haereses" St. Epiphanus, Bishop of Salamina (Cyprus): "The Panarion" (All the schisms).