The Church teaches (CCC 841) that "The plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Muslims; these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind's judge on the last day."
These days, many people are inclined to be skeptical of this as a specimen of post-Vatican II ecumania and indifferentism.
The problem with this view is that it is emphatically nothing new in the Catholic tradition to see Islam as worshipping the same God we do. Case in point, modernist indifferentist heretic Pope St. Gregory VII, writing to the Muslim Sultan of Bougie in North Africa in 1076:
For there is nothing which Almighty God, who wishes that all men should be saved and that no man should perish, more approves in our conduct than that a man should first love God and then his fellow men … Most certainly you and we ought to love each other in this way more than other races of men, because we believe and confess one God, albeit in different ways, whom each day we praise and reverence as the creator of all ages and the governor of this world.
Nor is the appeal to the sins of Radical Islamists much help in proving that the God of Islam is not the same God we worship. We Catholics, who have been told for decades that sinful Mafiosi, or anti-semites, or abusive priests somehow render Catholicism idolatrous should know better. Sin proves that we are sinners. It does not prove that we believe in "another god". So we are left with the search for a theological argument to show that Muslims worship some other god. Typically this boils down to citing 1 John: "No one who denies the Son has the Father" (1 John 2:23).
Now there is no question that, Muslims deny some key truths of the Faith including the Trinity and the deity of Christ–but then, so do Jews. Yet only the most extreme Reactionary Dissenter would conclude from this that these fellow monotheists worship "another god." So what gives?
Exclusivist Catholics seem to me to be ultimately erring in two ways in trying to reject CC 841. The first error we will discuss this week, the second next week.
The first error is called "salvation by intellectual works" It is the idea that God will not accept somebody who does not have all their intellectual conceptions of God in perfect shipshape. But if this were the case, then it would be mighty tough for any of the worthies of the Old Testament to be saved. Abraham, Isaac, and Jacob never professed faith in the Trinity. Isaiah held no doctrine of Transubstantiation. Yet we know that they are in Heaven.
Analogously, many people today, through no fault of their own, "reject" Catholic teaching (due to who-knows-what sort of familial and cultural baggage that keeps them from seeing Jesus as the Church sees Him). The Exclusivist who simplistically cites "No one who denies the Son has the Father" but who does not take into account the Church's teaching on culpability is simply practicing another form of Fundamentalism. Yes, Jesus said, "He who is not with us is against us." But he also said, "He who is not against us is for us." The Muslim whose only conception of the Trinity is that Christians believe God the Father had physical relations with the Blessed Virgin is not sinning by rejecting this stupid notion. Indeed, he is (if he but knew it) affirming Catholic belief. If he never hears actual Trinitarian teaching in a way he can comprehend, and tries his best to be faithful to the light he has by living a life of charity, our faith hold out hope that he will be saved by the grace of Christ.
It is worth noting that the description of judgment in the parable of the Sheep and the Goats says nothing about having figured out correct doctrine. It is all about people responding as best they can to the light they have. Indeed, the mark of the saved sheep in that parable is surprise. By Jesus' account, none of his followers among the "nations" (those outside the visible covenant community) had the slightest idea they were serving him: "Then the righteous will answer him and say, 'Lord, when did we see you hungry and feed you, or thirsty and give you drink? When did we see you a stranger and welcome you, or naked and clothe you? When did we see you ill or in prison, and visit you?'" (Matthew 25:37-39).
This is why the counsel of the Church to those who are too eager to know who is and is not saved is to remember, "We know where the Church is. We do not know where it is not."