Many Muslims assume that the Christian belief in the Trinity rests solely on 1 John 5:7 as it reads in the King James Version, "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." Christians have long understood and accepted that this verse was spurious and not part of the original text of 1 John. This is openly admitted by all New Testament scholars. In my debates I am often asked by Muslims about why I believe in the Trinity if 1 John 5:7 is spurious or not original. My answer has always been, that first and foremost, the Trinity does not rest on 1 John 5:7 nor does it rest on any one verse in the Bible. It is based on the totality of the witness of Scripture. Christian do not just believe in sola Scriptura (Scripture alone), they also believe in tota Scriptura (all of Scripture), the testimony of all the Scriptures.
In this video interview a Muslim expects Dr. Bart Ehrman to deny the Trinity mainly because of the questionable text of 1 John 5:7. You will notice Dr. Ehrman gives the same response I have always given. What is surprising, is Dr. Ehrman goes on to defend the doctrine of the Trinity! Please pray that Dr. Ehramn will find salvation in Christ alone.