For someone whose entire outlook is shaped by the idea of tradition and devoted to its explication such that he identifies himself as both traditional Muslim and traditionalist in general, it is difficult to consider Dr. Nasr as somehow untraditional. By this accusation I understand you to believe that his perspective is not in conformity with Muslim orthodoxy, and therefore that it is heterodox in nature. I can understand how you might reach this conclusion if you do not share his vision of the spectrum of the Islamic tradition and the dimensions which are embraced by Muslim orthodoxy. These are most easily divided into two and include the exoteric, which refers to the legal, historical, and theological aspects, and the esoteric, which concerns the spiritual path of Sufism, much of Islamic philosophy, and the formulations of theoretical gnosis derived from metaphysical realization or what Dr. Nasr refers to as scientia sacra, the science of the real.
The esoteric dimension necessarily embraces the exoteric such that, for example, Sufis are firmly grounded in the sharia and fully participate in the legal dimensions of their religion. The converse is not true, however, as the exoteric dimension does not necessarily embrace the esoteric. Considering that Dr. Nasr's perspective and many of his teachings are based on an understanding of metaphysics, he may sometimes appear to be at odds with some strictly historical and theological viewpoints, but without ever deviating from Muslim orthodoxy.
Considering that there are many different and sometimes conflicting historical, legal, and theological perspectives, even at the exoteric level, there is the possibility of an even greater degree of conflict between the exoteric and esoteric. That this is not always the case is demonstrated for example by the teachings of al-Ghazali particularly his work The Revival of the Religious Sciences which gave much general credibility to esoteric formulations by rigorously demonstrating their confirmation in the Quran and recorded traditions. A thorough acquaintance with both Muslim traditions and Dr. Nasr's teachings demonstrates a similar situation. It is precisely this acquaintance that led William Chittick, a contemporary Muslim scholar who is admirably qualified to undertake such an assessment to make the following statement in his book Science of Cosmos, Science of Soul.
"In several of his works," Chittick wrote, "Nasr has explained the main principles of the traditional Islamic worldview ... his interpretation of the contemporary implications of Islamic thought are firmly grounded in the tradition, much more so than many of his critics would like to acknowledge. The fact that he does not always cite Muslim authorities, but instead is likely to refer to Frithjof Schuon or Ananda Coomaraswamy, cannot be taken as evidence that his views do not have the Islamic support that he claims." (p.78)
Your accusation is based on an assessment of statements that Dr. Nasr has made concerning Christianity that you believe are questionable and of which you have provided two examples. To paraphrase your comments we can say that your objections rest upon the following points:
 Dr. Nasr seems to rationalize the formulation of the Christian Trinity as a legitimate worldview.
 Dr. Nasr regularly makes reference to "Christ" which contradicts the Muslim view of Jesus the Messiah.
 Dr. Nasr adheres to the idea that Christianity was existent before Paul which is historically inaccurate as demonstrated in Acts 11, v.26.
As a final remark concerning Dr. Nasr, you stated that you believe his purpose in proclaiming that Christians worship the same God that Muslims do is to suppress the violent inner nature of most Christian societies and to prevent them from slaughtering us. It will be prudent to address this last point first both to dispel this highly polemical view of Christianity and to establish Dr. Nasr's perspective concerning religions as a foundation for the examination of your main points listed above.
We may state unequivocally that Dr. Nasr does not share this view of Christianity. This is demonstrated by statements made in The Heart of Islam where he wrote,
"It is easy for Muslims and Christians, or for that matter Hindus, Confucians, or Buddhists, to point to episodes of war in the history of other religions. The history of all societies, whether religious or secular, is replete with such examples, because human beings contain in their fallen state the seeds of strife and contention and take recourse in aggression and war, using for their cause whatever idea or ideology has the power to move people ... When we return to the teachings that are at the heart of all authentic religions, however, we see that the role of each religion is to seek to bring about peace and to accentuate those religious teachings that emphasize both heavenly and earthly accord, harmony, and peace." (p.217-218)
When taking the purpose of Dr. Nasr's teachings into consideration, especially in light of the great diversity of subjects, it becomes evident that there are a variety of possible motivations. When dealing with the exposition of religions other than Islam, however, there is a particular motivation that seems to gain emphasis. This consists of strengthening the Faith of society at large which has simultaneously become threatened by both the agnosticism of secular ideologies in the modern west and the destruction of religious homogeneity through the widespread encounter of alien religious forms. He developed the implications of this situation in a lecture entitled "Living in a Multi-Religious World" (recorded in The Essential Seyyed Hossein Nasr) and even went so far as to identify the extensiveness of this encounter of religions as the single most significant phenomena of the twentieth century.
"Much has been said about the new adventures of man in the twentieth century," he stated, "the age known for the use of atomic power and flight into space. However, I personally believe that there is in truth only one new experience of real significance which confronts twentieth century man, one which his ancestors did not face. That experience is not one of discovering new continents and even planets, but one of journeying from one religious universe to another ... a new situation whose main features and characteristics cannot be neglected by any intelligent person interested in the phenomena of religion or belonging to the world of faith." (p. 3,5)
A detailed inquiry into these characteristics would take us far beyond the scope of the present writing. An excellent overview and introduction to Dr. Nasr's thought including a systematic treatment of the subject of the contemporary encounter of religions can be found in The Essential Seyyed Hossein Nasr. We include a mention of this subject here only in order to shed light on his purpose for discussing Christianity in a work devoted to Islam that you referenced (i.e. The Heart of Islam) as well as indicating something of his approach to understanding the subject of Christianity.
In the lecture mentioned above he further stated that
"... it is necessary to go beyond the polemical position of the medieval theologians who lived in a homogeneous ... universe and who could afford to ignore the universality of revelation and the reality of religion in diverse forms. One cannot read the Bhagavad-Gita seriously in this day and age without becoming aware of the religious character of this text. Nor could any one in good conscience call it pagan ramblings. Therefore, the doctrines of the other religions which are now available in the form of sacred scripture, open metaphysical exposition, theological formulation, or inspired literature of one kind or another, convey a metaphysical, theological, and religious significance which must be taken seriously by men and women of good faith." (p.6)
Similarly, no contemporary Muslim can approach the Christian religion in good conscience and dismiss its liturgical traditions and theological formulations as mere pagan ramblings when their sacred character and theological integrity are so obvious. It is in this spirit of taking Christianity seriously that Dr. Nasr explains the doctrine of the trinity and holds it to be legitimate for Christians within the context of the Christian worldview without in any way compromising his integrity as a traditional Muslim.
Nevertheless, the concept of the Trinity is a particularly complicated subject and Islam does not necessarily reject it outright for as Sachiko Murata and William Chittick explain in the The Vision of Islam
"the Koran and classical commentaries offer plenty of room for a view of things that is full of subtlety and nuance ... To take a simple example, it is commonly held that the Koran rejects the Christian concept of the Trinity. Inasmuch as the Trinity is understood as negating tawhid this is true. But not all Christians think that the Trinity negates tawhid. Quite the contrary, most formulations of the Trinitarian doctrine are careful to preserve God's unity. If threeness takes precedence over oneness, then the Koranic criticisms apply. But among Christians, the exact nature of the relationship between the three and the one is a matter of recurring debate ... the actual Christian position is highly subtle, and few if any Christians would hold that they have faith in other than a single God." (p.169-170)
You also seem to hold dispute with the use of the term Christ stating that Muslims do not believe in Jesus the Christ, only Jesus the Messiah. The greek term christos and the Hebrew mashiach are equivalent terms both meaning "annointed" though the former has become associated with certain connotations which separate the Christian understanding of Jesus from that of Islam. In Christianity, Jesus is an avataric figure or incarnation of Divinity and within Islam he is a prophet, although according to the Quran he has a unique quality and function among prophets as well as a miraculous origin in confirmation of the Christian account of the nativity.
There is no denying the fact that there are certain irreconcilable differences between Christian and Muslim doctrines. This is not an issue of dispute. In fact, when Dr. Nasr is describing the nature of the Christian theological formulation in the passage that you quoted from The Heart of Islam, he is doing so in contrast to that of Islam. The passage in its original context may be considered as an Islamic intellectual response to Christianity. Read in this light, it supports Dr. Nasr's traditional character rather than challenging it. It also demonstrates the comprehensiveness of his understanding and the versatility of the metaphysical perspective which enables him to view Christianity both in its own terms and from within the perspective of the Muslim worldview. An expanded quotation of the passage reads as as follows,
"Paradoxically, the insistence of Islam upon God as the One and the Absolute has had as its concomitant the acceptance of a multiplicity of prophets and revelations, and no sacred scripture is more universalist in its understanding of religion than the Quran, whose perspective concerning the universality of revelation may be called 'vertical triumphalism.' In contrast, in Christianity, because of the emphasis on the triune God, God the one is seen more in terms of the relationality of the three Hypostases, what one might call 'Divine Relativity'; the vision of the manifestation of the Divine then became confined to the unique Son and Incarnation, in whom the light of all previous prophets was absorbed. In Christianity the vision is that of the Triune God and a unique message of salvation and savior, hence extra ecclesiam nulla salus (no salvation outside the church), whereas in Islam there is the One God and many prophets. Here is to be found the major difference between how Muslims have viewed Jews and Christians over the centuries and how Christians have regarded Jews and Muslims as well as followers of other religions." (p.21)
Finally, you take issue with Dr. Nasr's indication that Christianity existed prior to Paul. in reality, this is not an issue of contention because it is not a tenet of Islam that Christianity was invented by Paul nor is this idea even shared by most Muslims. Regarding the validity of the idea in itself, quite irrespective of its consideration in terms of Islam, we may also state that despite the fact that Jesus' disciples first took on the name of Christians in Antioch after Paul had begun his ministry, this does not in any way preclude the existence of Christianity prior to this regardless of whether or not it was referred to as such or manifested itself in exactly the same way. For it is possible, like early Sufism, to be a "reality without a name" and like Shi'ism, to take on a unique character and manifestation following the death of the founder without entering into the realm of heterodoxy.
It is important to realize also, that Dr. Nasr's view of religion is not limited to this consideration of historical unfoldment. For Nasr, religion is a metahistorical reality such that Christianity, for instance, not only predates Paul, but also possesses a timeless and eternal quality. Religion exists prior to its unfoldment in history and continues to exist long after its earthly presence has been withdrawn. In Knowledge and the Sacred he develops the metaphysical understanding of religion stating that
"each revelation is in fact the manifestation of an archetype which represents some aspect of the Divine Nature. Each religion manifests on earth the reflection of an archetype at whose heart reside the Divinity itself. The total reality of each tradition, let us say Christianity or Islam, as it exists metahistorically and also as it unfolds throughout its destined historical life, is none other than what is contained in that archetype ... Religions do not die since their archetype resides in the immutable domain and they are all possibilities in the Divine Intellect." (p.294-296)
It is my hope that these words do adequate justice to your objections while displaying a position that is not driven by superficial sentimentalism, that they demonstrate the traditional character of Dr. Nasr's teachings, and further explain some of his viewpoints in support of our ongoing discussions on Tradition in this forum.
As a closing remark, I would like to say that I believe that it is important for us as religious people living in the twentieth century to make an active effort to promote peaceful relations between ourselves, avoiding polemics whenever possible. I also believe that it is possible to view other religions honestly and respectfully without discarding or overlooking obvious differences. As Muslims, we have an even greater responsibility due to the universality of the outlook and message presented by our scripture and that has been traditionally held throughout the history of Islam. The Holy Qur'an teaches us,
"You will surely find that, of all people, the ones nearest in love to those who believe [in the Koran] are those who say: 'We are Christians.' That is because there are among them priests and monks, and they are not proud. When they hear that which has been revealed to the Prophet you see their eyes overflow with tears, in recognition of its truth ... " (5:82)
If we are honest with ourselves and conduct our lives in a manner that embodies the virtues of the Prophet Muhammad, peace and blessings be upon him, and the intellectual integrity of our scriptures and traditions, those who are nearest in love to us, the Christians, may come to recognize the truth in our religion as we recognize the truth in theirs.