So I was interested to read John Charmley's piece on the (always fascinating) All Along the Watchtower site. John does not duck the difficulties, and indeed links to a previous piece of his own in which he argues the case against the proposition. However, his more recent piece is more nuanced, and reconciles what the Church officially teaches with his own difficulties.
In short, it hangs on that clause I introduced earlier: in so far as they worship the God of Abraham, then they worship the same God as we (and the Jews, of course) do. Of course, they do so in a limited way, as, like the Jews, they have not accepted the fullness of Revelation in Our Lord Jesus Christ, and consequently know far less of God (eg they deny the Trinity) and how to worship Him.
Reading today's Epistle from St John set another thought going in my mind. 'Everyone who loves is begotten by God and knows God. Whoever is without love does not know God, for God is love.' So, in so far as the Muslim (or Christian, come to that) is truly seeking to live out of love, he knows (however imperfectly) God.
All that then reminded me of another attempt to deal with these difficult issues: C S Lewis' in The Last Battle.
Here Emeth, surely the type of a good Muslim in Lewis' thinking, encounters Aslan: I said, Alas Lord, I am no son of thine but the servant of Tash. He answered, Child, all the service thou hast done to Tash, I account as service done to me.
Lewis however, rejects the idea that we worship the same God: Lord, is it then true, as the Ape said, that thou and Tash are one? The Lion growled so that the earth shook (but his wrath was not against me) and said, It is false. Not because he and I are one, but because we are opposites, I take to me the services which thou hast done to him. For I and he are of such different kinds that no service which is vile can be done to me, and none which is not vile can be done to him. Therefore if any man swear by Tash and keep his oath for the oath’s sake, it is by me that he has truly sworn, though he know it not, and it is I who reward him. And if any man do a cruelty in my name, then, though he says the name Aslan, it is Tash whom he serves and by Tash his deed is accepted.
Forced to choose between Lewis and the teaching of the Church, I naturally adhere to the teaching of the Church. But I quote Lewis because I think his insight also applies. When a Muslim (or a Christian) does things in the name of God which are against love, it is not God whom he serves, but Satan.
As I said at the start, I am a simple soul. The Church teaches that Muslims worship the same God and I accept that. The Church also teaches that their understanding of God is limited, and that they need Christ for salvation, and I accept that. The Church also teaches that, in so far as Islam differs from Christianity (and that is a very substantial amount indeed), it is a false religion. And I accept that.
And mutatis mutandis, the same could be argued about all the Protestant denominations which reject aspects of the Catholic Faith, to unpack a little further what Archdruid Eileen says here.
Of course the state of any individual's soul is not mine (or yours) to judge.