[Full Court]

It certainly wouldn't be that he believes in Islam or believes that Islam is on a par with Christianity. If he believed either of these two things then he (a) wouldn't be the earthly head of the Christian faith and (b) wouldn't have approved the publication of Dominus Iesus, which asserts the salvific universality of Jesus Christ and the Church.

The Catechism of the Catholic Church

841 The Church's relationship with the Muslims. "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." 330

[aimhigh]

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,

This does not speak well of Catholic doctrine. The Muslims don't adore the merciful God, they adore the vengeful god.