Like many other religions, Islam contains a diversity of perspectives crystallized within numerous schools or divisions. These may differ according to legal rulings, religious doctrines, theological perspectives, mystical inclinations, and other possibilities. The Quran sanctions this diversity referring to the variance of opinions among the scholars as a mercy to us. Despite these outward differences, all of these perspectives maintain their identity as "Islam" through their adherence to the fundamental elements of the faith. Recently, there has been a collective effort centered around the Royal Court of Jordan to identify and explain these fundamental elements of faith shared by all Muslims. The result has been a series of documents such as The Amman Message, the Letter to Pope Benedict and other Christian leaders, and A Common Word Between Us and You, that are endorsed by Muslim scholars and leaders representing virtually all of the various schools and divisions of Islam.
Should it be necessary to speak on behalf of Islam as a whole, reference to these documents may enable us to adequately represent what most if not all Muslims believe. Concerning the teaching of Islam with respect to Jesus Christ for example, we find the following explanation in A Common Word,
"Muslims recognize Jesus Christ as the Messiah, not in the same way Christians do (but Christians themselves anyway have never all agreed with each other on Jesus Christ’s nature), but in the following way: …. the Messiah Jesus son of Mary is a Messenger of God and His Word which he cast unto Mary and a Spirit from Him.... (Al-Nisa’, 4:171). We therefore invite Christians to consider Muslims not against and thus with them, in accordance with Jesus Christ’s words ..."
Undoubtedly, many elements of Christian ritual and philosophy have derived from the traditions of Graeco-Roman Paganism including the Ancient Mystery Religions and the philosophies of Plato, Aristotle, and the Neoplatonists, among others. To understand the nature of this influence it is sufficient to recognize the unique destiny of those religions which possess a universal character, specifically Christianity, Buddhism, and Islam. As distinct from those that were destined for a particular people and so may be referred to as ethnic religions which nevertheless possess a degree of variation, these three universal religions possess an even greater degree of variation due to the assimilation of numerous cultural and ethnic customs and contingencies. When spreading to a new region these religions did not destroy the respective cultures but rather assimilated and transformed those elements of the native tradition which did not conflict with the fundamental nature of the revelation and were capable of rendering support to it.
Thus, within India, China, Japan, and Tibet, Buddhism manifests a different character corresponding to the unique characteristics of the cultural and ethnic qualities and traditions of the region all the while remaining distinctly "Buddhism." We can make the same comments about Islam and Christianity within these very regions. Christianity was necessarily destined to assimilate various elements of Graeco-Roman culture due to its reception within and subsequent spread from a region of greek-speaking gentiles. Suffice to say that it is at present, following its subsequent global migration, not limited to these influences as is evident for example in the Protestantism issuing from the assimilation of germanic cultures.
There is a very curious anecdote in Knowledge and the Sacred which may explain how Islam can accept a religion such as Christianity, particularly of the Roman Catholic variety, despite a preponderance of pagan influences within certain of its manifestations. Dr. Nasr wrote,
“Revelation for Islam means the assertion of ‘al-tawhid’ and all religions are seen as so many repetitions in different climes and languages of the doctrine of unity. Moreover, wherever the doctrine of unity is to be found, it is considered to be of divine origin. Therefore, Muslims did not distinguish between religion and paganism but between those who accepted unity and those who denied or ignored it. For them, the sages of antiquity such as Pythagoras and Plato were ‘Unitarians’ (muwahhidun) who expressed the truth which lies at the heart of all religions.” (p.71)
Finally, in attempting to wrest Christianity from its place among the Abrahamic traditions, you are likely to find yourself alone. Even Shaykh Hamza Yusuf who you studied under in Fez adheres to this most common perspective. He wrote in an article on the Holocaust, "... our Abrahamic traditions of Judaism, Christianity and Islam have always been concerned with and seriously interested in epistemology, because each of these faiths have profound truth claims that need substantiation or 'believability.'"