Belloc cited the many beliefs and theological principles which Islam shares with Catholicism.
In The Great Heresies Belloc grouped the Protestant Reformation together with Islam as one of the major Heresies threatening the Church Universal..
Where Islam decisively diverges from Catholicism (and Christianity in general) is the denial of the Incarnation and all the sacramental life of the Church that followed from it.
Belloc cited the many beliefs and theological principles which Islam shares with Catholicism