This little book advises Christians involved in dialogue with Muslims to be clear about the meaning of words. The peril in such dialogues lies precisely in not sufficiently understanding that, under superficial agreement that there is only “one God,” and on some items of morality, Christianity and Islam differ about nothing less than the nature of God and of man. This book points out those differences, and how they are typically obscured.
Muslims worship a God whose fundamental nature is omnipotence. All of Allah’s other attributes, including mercy, are incidental to His sovereign will. All things and all events depend exclusively on His will. The sun rises, sets, and goes about its course instant by instant only because Allah wills it. Thus, as Reilly observes, for the devout Muslim “Everything is miraculous.” For the Muslim, to enquire about the secondary causes which Christians call “the laws of nature and Nature’s God, is blasphemous.
The omnipotent Allah cannot love, because, as Reilly quotes Islam’s foremost theologian al-Ghazali, “When there is love there must be in the lover a sense of incompleteness; a recognition that the beloved is needed for complete realization of the self.” Hence it is impossible for the wholly transcendent God to love creatures that are wholly different.
So transcendent is Allah, so inferior is man, that Islam deems doubly blasphemous the the statement in the Book of Genesis’ that God created man in His own image. Man in the image of God supposedly violates First Commandment’s assertion of God’s supremacy, while the very notion of an image of God violates its prohibition of making images of the things of heaven.
Christianity’s central tenet, that God took on human flesh and dwelt among us in the person of Jesus Christ, the Second Person of the Holy Trinity, violates Islam’s central tenet that God has no son, no “associate,” no “partner,”-- expressions which, Reilly points out, the most authoritative assembly of Muslim scholars ever convened used 15 times in a brief statement that defined their relationship with Christians.
In short, it is not strictly true that Muslims and Christians worship the same God.
Nor is it true, as Reilly points out, that Muslims understand humanity as do Christians. The Islamic scholars’ rendition of the Great Commandment – love of God and love of neighbor – that they claim to share with Christians -- uses a word for neighbor that refers to people close by rather than the alternate, which refers to humanity. But, as Reilly notes, “there are no Qur’anic citations” commanding love of neighbor. Hence
“strictly speaking, there is no such thing as a non-Muslim neighbor in Islam.”
Because the theological differences are so profound, Christian and Muslim notions of human relations cannot be less so. Reilly shows this by describing the intellectual controversy set off by pope Benedict XVI’s Regensburg speech of 2006. Benedict argued that, since God is logos, reason itself, and man is in His image, men may rightly lead one another only through persuasion. This view of God is consistent with Greek philosophy.
A substantial part of Reilly’s book describes a controversy that occurred in the 9th century AD between Muslims who had accepted Greek philosophy  -- and with it this view of human relations – and Muslims who asserted that the only authority comes from the Qur’an’s revelation. Hence, argued the latter, because right consists neither more nor less than of submission to that authority, any reference to reason is illegitimate because subversive.
The problem, as Reilly shows, is “the ontological status of reason.” In this regard he cites a Muslim theologian, Arif Nayed, who tries to finesse the issue by admitting the importance of reason in human relations, while denying the identity of logos with God. Reilly may be too quick in dismissing this admission because it is insufficiently grounded in ontology.
This little book culminates, as it should, in brief reflections on the practical side of Catholic-Muslim dialogue. In this regard, it cites the work of the United States Conference of Catholic Bishops (USCCB) ,which produced, most notably, a guide to understanding the problems of marriages between Catholics and Muslims, titled “Marriage.” Reilly writes:

To its credit, Marriage provides a good deal of sober advice in presenting the very substantial religious and cultural differences that any Catholic-Muslim marriage would have to overcome (which occurs only in the case of a Catholic woman marrying a Muslim man, as Muslim women are forbidden from marrying Catholic men). While it refers to the different status of men and women in Islam, it does not deal with the issue of polygamy, the statutory inferiority of women in shar‘ia, or of the Qur’anically sanctioned use of violence against a disobedient wife. Perhaps this is because of the illegality of polygamy and domestic violence in the United States, along with the legal inapplicability of shar‘ia, or possibly out of a desire not to offend – though this omission might leave the Catholic wife of a Muslim husband, who returned with him to a Muslim country in which polygamy is legal, in a most unfortunate situation.

That a joint effort of Catholics and Muslims would pull punches in describing sensitive subjects is easy enough to understand. It is also a problem. Reilly cites “Dr. John Borelli, who was the associate director of the Secretariat for Ecumenical and Interreligious Affairs at for 16 years:… ‘There is a need for each side to comprehend as much as possible and understand correctly the beliefs of the other side in their own terms.’ As one might imagine, this can take a life- time of learning to do correctly.” Angelo M. Codevilla is Professor Emeritus at Boston University.