How can Allah be the one true God?

The word Allah is the Arabic equivalent to the English word God. The word Allah is used by Arabic-speaking Christians and Jews as well as Muslims. Although they have an incomplete understanding of the nature and revelation of God, Muslims worship the same God whom Christians and Jews worship.

The Church regards with esteem also the Muslims. 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. (Nostra Aetate 3)