It was the great Islamic theologians and thinkers — among them al-Farabi, Ibn Sina (Avicenna), and Ibn Rushd (Averroes) — who recovered the classical tradition of philosophy, leading the West out of the Dark Ages.I gave a talk on this subject once at US Central Command. Avicenna in particular is important to Christianity because of a mistake that was very helpful to Aquinas. Avicenna inherited a work of Plotinus' that had been wrongly titled The Theology of Aristotle. Plotinus' philosophy, which we call Neoplatonism, is really not at all similar to Aristotle's metaphysics (which is the closest he ever gets to a theology). Avicenna devoted himself to harmonizing these two doctrines because he thought that doing so was necessary to understand the deeper, hidden message of Aristotle that somehow these two quite different works were both attempting to convey.
Maimonides, one of the greatest Jewish thinkers in the past 1,000 years, was also deeply indebted to them. Throughout his masterwork, The Guide for the Perplexed, he is in constant dialogue with the Mutakallimun, or the Muslim Kalamists. Even his great religious law code, the Mishneh Torah, was inspired by Sharia codes. Maimonides, in turn, influenced Christian thinkers like Aquinas. Thus, both Judaism and Christianity are deeply indebted to the thinkers of Islam.
Avicenna's metaphysics was therefore quite useful to Aquinas, who inherited Aristotle from the early phases of the Christian reconquest of Islamic Spain. Aristotle's works were scientifically far superior to what existed at the time, but they were philosophically incompatible with the Neoplatonic thought that had influenced many early leading thinkers of the Church. Avicenna provided the way forward: he showed how to harmonize the new Aristotelian science with the Neoplatonic-influenced interpretations of Christian writings that were important to several early saints (even Augustine).
All of that is right. And yet if you turn to the back of Avicenna's metaphysics -- I have a copy right handy -- you find plenty of the following: "As for enemies and those who oppose the law, [the legislator] must decree waging war against them and destroying them -- after calling on them to accept the truth -- and [decree] that their property and women must be declared free for the spoil." That's exactly ISIS's position, you'll notice.
Likewise: "The same applies to people that are far removed from acquiring virtue. For these are slaves by nature -- as, for example, the Turks and the Negroes..."
So, do we owe a lot to Avicenna? Yes. Does that imply that our views and Islam's are fully compatible? No. Some small objections remain.