Here it should be emphasized that a number of factors could have caused a receptivity to people who belonged to other religious traditions, even rival and competing ones: religious indifference and ignorance of one’s own tradition; sheer pragmatism that grew from everyday human interaction; the realities of governing an empire that was overwhelmingly non-Muslim; and, in the case of a political elite that was made up of Muḥammad’s staunchest opponents and their descendants, a possible lack of deep religious commitment and conviction. Though in practical terms, an openness that resulted from a receptivity rooted in one (or some combination) of these is much the same as that which would have stemmed from a consciously inclusive ideology, we should not mistake an inclusiveness that stemmed from the first set of factors with an openness that grew from an explicitly ecumenical ideology. For this reason, the image of an ‘ecumenical’ early Islam, such as has recently been advocated, is unpersuasive.

1. As Jack Tannous states, Any view of an ‘ecumenical’ Believers movement or of an Islam that is not religiously distinct from Christianity or Judaism in the seventh century also has to explain pieces of evidence that suggest the ideology underpinning Muḥammad’s community, even in this period, could be decidedly particular, sectarian, and critical of both Judaism and Christianity. So, for instance, the Qur’ān criticizes the Christian belief in the divinity of Jesus (5:116) and has Jesus tell Christians he wanted them to worship God (5:117). It orders Christians to not speak of ‘three’ when speaking of God and criticizes the idea that God might have offspring (4:171). It also calls speaking of Christ as divine disbelief (5:72); calls speaking of God as three unbelief and promises painful torture for those who persist in doing so (5:73); calls Christ only an apostle (5:75); denies that he was killed or crucified (4:157); criticizes Christians for calling Jesus the Son of God and Jews for calling Ezra the Son of God (9:30); accuses rabbis and monks of blocking people from the path of God (9:34); and states that those who pursue a religion other than Islam will be counted among the losers in the next life (3:85). The Children of Israel are accused of distorting Scripture and forgetting some of what they have been told (5:13; cf. 4:46, 5:41); Christians have also forgotten some of what they have been reminded of and God has caused dissension among them (5:14). Moreover, the Qur’ānic Messenger has come in part to clarify what the People of the Book have hidden in their Scriptures (5:15). The People of the Book are portrayed as skeptical, asking Muḥammad to bring a Scripture down from heaven (4:153), and members of the Prophet’s community are told they will surely hear much abuse from those already in possession of a Scripture and from those who associate with God (3:186). Those who believe are ordered not to take Jews or Christians as allies (5:51). If the People of the Book had believed, the Qur’ān affirms, it would have been good for them; only a few believe, however, and most do not (3:110; cf. 3:199, 4:155, 5:66). Outside the Qur’ān, an ecumenical hypothesis needs to account for mid-seventh-century reports of Arab hostility to the Cross (see n. 83 and cf. also n. 84 in this chapter); the mid-seventh-century report that in the early 660s Muʽāwiya tried unsuccessfully to remove crosses from coins (n. 91 in this chapter); and the mid-to-late-seventh-century report that Muʽāwiya wrote to Constans II, calling on him to abandon Christianity and turn from Jesus to worship the God of Abraham (see Thomson, trans., The Armenian History attributed to Sebeos, vol. 1, p. 144).
2. As we have seen, however, many of the same behaviors that can be pointed to as occurring between Christians and Muslims in the early period of Arab rule can also be seen in the relations between different Christian confessional groups in the post-Chalcedonian world. And such behaviors continued to occur in the Middle East throughout the Middle Ages. So far as intergroup relations were concerned, the seventh century was not a unique moment in Middle Eastern interconfessional interaction in the first millennium AD. It was utterly typical. Like traditional approaches to Christian-Muslim relations, which have focused heavily on doctrinal matters such as the Trinity and the Incarnation, the notion of an ‘ecumenical’ early Islam, too, is one-dimensional and lacking in texture. It does not acknowledge the layering of knowledge that characterizes religious communities, nor does it take into account the consequences of such a layering. Rather than assuming, as many traditional approaches implicitly do, that all Christians and Muslims in the seventh and eighth centuries were maximally informed about and concerned with this or that matter of theological speculation, such an approach assumes that all Muslims in this period were maximally informed about the content and implication of the Prophet’s message and takes as reflective of the actual nature of what Muḥammad taught behaviors that can more easily be explained by factors such as mass conversion, religious ignorance, selective religious regard, or religious indifference.

If, for the sake of argument, it is granted that the Prophet’s original message was one of tolerant and inclusive monotheism, the question of the extent to which most members of his community will have known about and internalized this message and used it as the basis for personal conduct will persist; for the challenges of catechesis in a community that was largely comprised of mass converts and simple adherents will also persist, regardless of whatever it is one wants to argue the Prophet’s original message actually was. Saying that the early medieval Middle East was a world of simple believers is not the same as saying that it was a world of Believers who had consciously articulated views of religious inclusion toward other monotheists and a strict code of personal piety, or that, following certain models of the Christian-Jewish ‘parting of the ways,’ it would take two to three centuries before Christianity and Islam came to have distinct boundaries in the Middle East (Penn, Envisioning Islam, p. 182).

1. Jack Tannous in his book (The Making of the Medieval Middle East) sees the Qur’an’s attacks on the Christian doctrine of the Trinity as incompatible with this hypothesis, an objection that others have brought as well.
2. Shoemaker Criticism?

1. Shoemaker doesn’t do a very good job criticizing Tannous IMO. He rather claims its’ a strawman without actually proving on how it’s a strawman.
2. Shoemaker actually later concedes with Tannous’ point:

As the community excluded belief in Christ’s divinity and a triune God, Christian Believers must have faced a choice: either break with Muhammad’s community in favor of the Christian faith or adjust their beliefs about Jesus according to the evolving nature of the new faith that they had embraced. And Shoemaker believes that the so-called Trinitarian Christians had to leave Trinitarianism and affirm only one God while following the Prophet.