Aslan was the Lion.
Compiled a bit of text on the subject from various sources below. No idea how accurate it is but it is repeated in several places.
CS Lewis believed Islam to be a mixture of truth and falsehood. That's what he meant by calling it "the greatest of the Christian heresies."
This originated from John of Damascus’ De Haeresibus (Of Heresies) who was the earliest extant robust evaluation of the Islamic faith. John’s use of the word “heresy” here amounts to an admission that Muslims, despite their many departures from Christian orthodoxy, are in the same world of discourse as Christians, although the disparities are substantial. The wider Christian world did not discount John’s categorization but adopted and continued it—most famously in Dante’s Divine Comedy, where Mohammed is placed in the infernal circle of schismatics because he sundered Christian unity in Asia and Africa with his Arabian heresy.
Lewis was a committed Anglican who upheld a largely orthodox Anglican theology, though in his apologetic writings, he made an effort to avoid espousing any one denomination.
Accordingbto Hartmut Bobzin in The Qur’an as Text (Brill, 1996), the idea that Islam is a heresy, even the “epitome of all heresies,” and the Qur’an “an awkward figment of Satanic imagination” and a “treasury of heresy,” would become popular not only in eighth-century Byzantium, but also in twentieth-century Europe. Many thought the Qur’an held nothing but a mixture of old heresies that earlier church verdicts had refuted.
At the time of the Reformation, Martin Luther himself declared that Mohammed was one of the heads of the anti-Christ (the pope, of course, was the second head!), and he sought therefore to strengthen preaching against the Islamic “temptation.” Catholics, for their part, declared that Islam was merely ritualism devoid of sacraments and mystery. People often heard the idea that the Islamic faith forced Muslims to submit blindly to a tyrannical overlord.
Lewis writes in his raw and beautiful book, A Grief Observed, which chronicled his heartbreak after losing his wife, Joy Davidman, to cancer in 1960. Many have read this text and have interpreted that Lewis might have lost his Christian faith after losing Joy.
