Replies

Belloc has an essay where he calls Islam the first external heresy of Christianity and he makes a good case, so I'm less sure than either of you as to the absolute true nature of Islamic monotheism. Islam venerates Jesus, Mary and Joseph for example. It treats the infidel and heretic who are Jews or Christians differently (and better) than pagans, for another example. All that said, I think it would be sinful for me as a practicing Catholic to attend a prayer service in another faith. I am certain it would be were I to worship "their" God, if their God was NOT the Triune God. Since I am reasonably sure that Islam rejects the Trinity, I am reasonably certain worship in an Islamic Center's services would be sinful for me. Even if I weren't certain, doubt would clearly be present and moral doubt should be enough to stop us from action to prevent us from the clear possibility of mortal sin.

As Sink noted above, His Holiness kissed the Q'ran. Perhaps hierarchs are bound by different rules than I was taught, or perhaps I misunderstood that part of the Ten Commandments about "No other God".

2084 God makes himself known by recalling his all-powerful loving, and liberating action in the history of the one he addresses: "I brought you out of the land of Egypt, out of the house of bondage." The first word contains the first commandment of the Law: "You shall fear the LORD your God; you shall serve him.... You shall not go after other gods." God's first call and just demand is that man accept him and worship him.

2085 The one and true God first reveals his glory to Israel. The revelation of the vocation and truth of man is linked to the revelation of God. Man's vocation is to make God manifest by acting in conformity with his creation "in the image and likeness of God":
There will never be another God, Trypho, and there has been no other since the world began . . . than he who made and ordered the universe. We do not think that our God is different from yours. He is the same who brought your fathers out of Egypt "by his powerful hand and his outstretched arm." We do not place our hope in some other god, for there is none, but in the same God as you do: the God of Abraham, Isaac and Jacob.

2086 "The first commandment embraces faith, hope, and charity. When we say 'God' we confess a constant, unchangeable being, always the same, faithful and just, without any evil. It follows that we must necessarily accept his words and have complete faith in him and acknowledge his authority. He is almighty, merciful, and infinitely beneficent. Who could not place all hope in him? Who could not love him when contemplating the treasures of goodness and love he has poured out on us? Hence the formula God employs in the Scripture at the beginning and end of his commandments: 'I am the LORD.'"

Belloc has an essay where he calls Islam the first external heresy of Christianity and he makes a good case

There is a certain validity to this viewpoint, but only within a very limited context of "heresy". Mohammet merely borrowed some names (and a very distorted history) from local Jews and heretical Christians that he encountered. The "source material" for the vast majority of the religion is local, however (for example, using "Allah", a local moon god and minor member of the collection of idols worshipped at the Kaaba before Islam). While there are aspects of Judaism and Christianity that are rendered in the pages of the Quran, they are done so in a manner that is historically laughable (confusing people born centuries apart as if they were the same person, replacing the covenant promises to Isaac with Ishmael, having "Samaritans" suggest making the golden calf during the Exodus, and other silly mistakes that no literate author would make).

Islam venerates Jesus, Mary and Joseph for example.

Not really. It venerates Isa and Miriam. The Isa of Islam is not Jesus, the Son of God, but just a prophet, and Miriam the mother of Isa is not Mary, since she is also the sister of Moses (where Mary the mother of Jesus was born centuries later).

Allah has no Son (but did have three daughters, before some later revision of Islam), and was worshipped as one of a member of a pantheon of deities at the Kaaba long before Mohammet came along and invented Islam (destroying the other 359 idols at the Kaaba so that worship would focus solely upon the "new" monotheistic deity Allah).