But, under the traditional Catholic approach, that is incorrect.
In Catholic theology, when people worship one God–like the Jews or Muslims–it is considered to be the one true God. In Acts 17:23,
St. Paul tells pagans they are worshipping the one true God:
“For passing by, and seeing your idols, I found an altar also, on which was written: To the unknown God. What therefore you worship, without knowing it, that I preach to you.”
Here are some more sources for this idea that Muslims worship the one true God:
Pope St. Gregory VII (11th century) to a Muslim prince: ‘Almighty God, who wishes that all should be saved and none lost, approves nothing in so much as that after loving Him one should love his fellow man, and that one should not do to others, what one does not want done to oneself. You and we owe this charity to ourselves especially because we believe in and confess one God, admittedly, in a different way, and daily praise and venerate him, the creator of the world and ruler of this world.’
Catechism of Pope St. Pius X : 12
Q. Who are infidels?
A. Infidels are those who have not been baptised and do not believe in Jesus Christ, because they either believe in and worship false gods as idolaters do, or though admitting one true God, they do not believe in the Messiah, neither as already come in the Person of Jesus Christ, nor as to come; for instance, Mohammedans and the like.
Catholic Encyclopedia: As in ecclesiastical language those who by baptism have received faith in
Jesus Christ and have pledged Him their fidelity and called the faithful, so the name
infidel is given to those who have not been baptized. The term applies not only to all who are ignorant of the
true God, such as pagans of various kinds, but also to those who adore Him but do not recognize
Jesus Christ, as Jews and
Mohammedans.
Also see this chapter on Islam in
Hillair Belloc’s "Great Heresies."
ewtn.com/library/HOMELIBR/HERESY4.TXT
Second Vatican Council: The Church regards with esteem also the Moslems. They adore the one God, living and subsisting in Himself; merciful and all- powerful, the Creator of heaven and earth, who has spoken to men; they take pains to submit wholeheartedly to even His inscrutable decrees, just as Abraham, with whom the faith of Islam takes pleasure in linking itself, submitted to God.