Christians should be aware of the Lost Gospels of Adam and Eve, Testament of Solomon, Gospel of Nicodemus, Letters of Herod and Pilate, The Infancy of Jesus Christ, 1 and 3 Enoch, some knowledge of the Quran and possibly even the Apocalypse of Abraham. The Quran backs up the contents in the Testament of Solomon, Enoch and Infancy of Jesus. The Quran purpose is a final revelation which backs up the Gospel and Torah. A final book of scripture is revealed in Revelation 10. This means scripture would be incomplete until the seventh angel revealed the "scroll" from heaven (Rev 10). Much of the Quran covers esoteric subjects of 7 heavens, hell, eternal torment, Day of Resurrection, Gardens of Paradise, sitting on Thrones etc. These are essentially Christian Jewish concepts.
Coming from a radical fundamentalist church which viewed the Canon of Scripture as the only authority revealed to mankind then having to answer questions on gospels left out the Canon was a somewhat difficult process. We just accept the Council of men who determined what books were authority, these were "pagans" in their Biblical approach connected to the ruling Catholic church. Mystical books which talked about 7 heavens were rejected. Books with Jewish themes automatically rejected. Stories of Jesus' childhood were also rejected since it did not conform to existing theology.
Muslims err by rejecting the existing scriptures (Old and New Testament). Christians err by not accepting the Quran (since scholarship can prove its Jewish Christian origins), the Jews err by not accepting the New Testament and Quran. Those who refuse to believe the Quran is a book of authority then can you explain how the Prophet Mohammed knew about ascending into the 7 heavens (a Jewish concept of mysticism called merkabah), how did the Prophet Mohammed know about the Gospel of the Infancy of Jesus? How did the Prophet Mohammed know about the Testament of Solomon since he mentions Solomon putting the jinn (devils) to work and using them in battles? Christians reject the Gospel of the Infancy and Testament of Solomon yet Prophet Mohammed knew about these and were incorporated into the Quran. Those who believe Allah is not the God of Israel, clearly explain to me who is he?
The Christian Churches of God has done research on the Quran and would urge every true Sabbatarian who is interested in biblical truth to listen to the audio and read the articles on Islam. Wade Cox is able to reconcile the Islamic festivals with the Hebrew calendar, the rejection of the Sonship of Jesus can be explained and the command to observe the Torah (Taurat) is written throughout the Quran. The rejection of the crucifixion of Jesus can be explained by Islamic scholar Gabriel Sawma.
The Apocalypse of Abraham
The Apocalypse of Abraham is a pseudepigraphic work (a text whose claimed authorship is unfounded) based on the Old Testament. Probably composed between about 70–150 AD, it is of Jewish origin and is usually considered to be part of the Apocalyptic literature. It has survived only in Old Slavonic recensions—it is not regarded as authoritative scripture by Jews or any Christian group.
The text of the Apocalypse of Abraham has been preserved only in Slavonic; it occurs in the Tolkovaja Paleja (or Explanatory Paleja, a Medieval compendium of various Old Testament texts and comments that also preserved the Ladder of Jacob). The original language of this text was almost surely Hebrew: it was translated into Slavonic either directly from Hebrew or from a lost intermediate Greek translation. The whole text survives in six manuscripts usually gathered in two families: the main manuscript of the first family is referred to as S edited by Tixonravov in 1863, while the main manuscripts of the other family, which preserve the text integrated in other material of the Tolkovaja Paleja, are referred to as A,and K. The first English translation appeared in 1898 in the LDS magazine Improvement Era, and another notable English translation was produced by G.H. Box and J.I. Landsman some twenty years later."
A copy of The Apocalypse of Abraham can be downloaded at http://www.marquette.edu/maqom/box.pdf