[This post is dedicated to Naif-Al-Mutawa.--ES]
Abraham was neither a Jew nor a Christian, but he was one inclining toward truth, a Muslim [submitting to Allah]. And he was not of the polytheists.--Quran 3.67
Bowersock's The Throne of Adulis, which I read during the last few days, gives a glimpse of great power politics in Arabia just before the rise of Islam. It describes the (temporary) conquest of parts of Arabia by Christian Africans (from Axum or modern day Eritrea and Ethiopia) over the Jewish Arabs and their Kingdom of Himyar in present day Yemen (see Bowersock's blog-piece here). Bowersock is a very judgmental writer ("bizarre but militant kingdom in Himyar" occurs both in the blog post and in the book, which appeared with OUP), and he does not always explain his judgments. I also don't like that Bowersock offers little by way of cultural/anthropological context to his political treatment. But the story he puzzles together from various sources is fascinating.
Near the end of his narrative he calls attention to the passage quoted at the top of this post, and he offers an interpretation of it in light of the political history he has offered. Bowersock uses the koranic verse in a larger argument to conclude that "turning away from polytheism" does not entail belief in Islam. It echoes an important earlier scene in the book (71), where Bowersock had called attention to a "striking feature" of the conversion (by the Axum king) from polytheism to a Christianity initially without Christ.
In the quoted koranic verse Abraham is clearly some kind of monotheist. On some translations (as the one above), Abraham, who predates historical Judaism and Christianity, is portrayed favorably as a Muslim avant la lettre. If I understand him correctly, Bowersock claims, by contrast, that while he agrees that Abraham is portrayed favorably, the combination of haneefan musliman (transliterated) is unusual, because haneefan tends to designate pagans. So, on Bowersock's account we shouldn't treat the verse as offering Abraham as such Muslim avant la lettre.
Sadly, I don't read Arabic and I am not inspired, so I have no way of being an authority on the matter, which gets explosive in polemical context anyway.
Even so, Spinoza offers an alternative option logically available to a reader of the Quran. In chapter 2 of the Theological Political Treatise, Spinoza, too, treats Abraham as a special case. In fact, Spinoza treats Abraham, first, as deficient in crucial knowledge about God; that is Abraham does not know that -- quoting Curley's translation -- "God is everywhere and that he foreknows all things." Second, he insists (with a nod to Genesis 18:19) that whatever Abraham may have observed in his household (and we are told it is God's law), Abraham obeyed the public laws and rites under Melchizedek's kingship (TTP chapter 3). So, Spinoza inscribes onto Abraham a public/private distinction that allows Abraham to have true faith in God without being Jewish, not because the Tora had not been revealed yet, but rather because before God had not yet "founded the Nation of Israel."
So, for both Spinoza and the Quran Abraham can have the proper mold (or true spirit) of X without being in some other sense X. Interestingly enough, shortly after the passage I quote, the Quran goes on to say in ecumenical spirit, "We have believed in Allah and in what was revealed to us and what was revealed to Abraham, Ishmael, Isaac, Jacob, and the Descendants, and in what was given to Moses and Jesus and to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [submitting] to Him." (3.84) So, one can be Muslim in some non-trivial sense, without the benefit of revelation or even the revelation of Quran (see also 3.95).*
Now, one might think that my bringing Spinoza's and the Quran's analysis of Abraham together in this way is fanciful (hopefully not blasphemous), because one might think that there is a crucial difference between founding a polity and revealing a holy book. (Not to mention that Spinoza never considers Abraham as a Muslim.) But as it turns out, the life of the Prophet Muhammad reveals that founding a polity and revealing a holy book go quite well together. In fact, Spinoza also thinks (rather blasphemously) that in Israel's case polity-founding and the revelation of the Tora go together in the hands of Ezra (recall here and here).
Moreover, the idea that one treats a historically distant predecessor as a unique anticipation or exemplar, distinct from others and that does not quite fit taxonomic categories, has an unexpected afterlife in the treatment of the Scriptural Jesus in Chapter 1 of Spinoza's Theological Political Treatise:
for a man to perceive by the mind alone [III/21] things which are not contained in the first foundations of our knowledge, and cannot be deduced from them, his mind would necessarily have to be more excellent than it is, and far superior to the human mind.
 So I do not believe that anyone else has reached such perfection, surpassing all others, except Christ, to whom God revealed immediately – without words or visions – the conditions which lead men to salvation.--Spinoza (in E. Curley's translation).
So, as Spinoza puts it in Chapter four, Christ "perceived truly, or understood the things revealed...Christ, therefore, perceived the things revealed truly and adequately...no one is made blessed unless he has in himself the mind of Christ." Now, my talented student, Jo Van Cauter, is working out the details of what this precisely means about Spinoza's views (on salvation, blessedness, political theology, etc.). But in these passages Spinoza treats an image of Christ as a kind of philosophical exemplar that anticipates the teaching of the Ethics.
These days Spinoza is much beloved, especially by those who wish to use him as a secular saint and a figure to rally around against the Muslims in our midst. He is said to have inaugurated our path toward Enlightenment and pushed the march of history away from superstition. Leaving aside the roots of Spinozism in Ibn Rushd (go read Fraenkel), perhaps mediated via Gersonides (Klein), Spinoza, who knew a thing or two about the significance of exemplars, allows in chapter 5 of the Theological Political Treatise, that it is quite possible to read the Quran and have "salutary opinions and a true manner of living," and thereby being "absolutely blessed and really" have what he calls "the Spirit of Christ" in oneself.**
*At 3.3 the Tora and Christian Gospel are assimilated to the Quran.
**Yes, in context Spinoza is making another point at the expense of Christian Scripture.