Saint George of San Saba (died A.D. 852): "Mohammed was a disciple of the devil, and his followers are in a state of perdition." (In "Victories of the Martyrs" by Saint Alphonsus)
Blessed Nicholas Talvilich (died 1391): "You Mohammedans are in a state of everlasting damnation. Your Koran is not God's law nor is it revealed by Him. Far from being a good thing, your law is utterly evil. It is founded neither in the Old Testament nor in the New. In it are lies, foolish things, buffooneries, contradictions, and much that leads not to virtue and goodness but to evil and to all manner of vice."
The Church makes a distinction between heretics of defection and heretics of descent. A heretic of descent who has never known the Catholic religion may be justified through baptism of desire.
Fr. John Hardon THE QUESTION AND ANSWER CATECHISM
IS BAPTISM OF WATER NECESSARY FOR SALVATION? It is commonly taught by the
Church that baptism of water is necessary for salvation for those who have
not reached the use of reason.
WHAT IS BAPTISM OF DESIRE? Baptism of desire is the implicit desire for baptism of water by a person who makes an act of perfect love of God,
based on faith and with a sincere sorrow for one’s sins. Such was the case
in the Acts of the Apostles, when Peter encountered pagans who, moved by
the grace of the Holy Spirit, proclaimed the greatness of God. “Peter
himself then said, ‘Could anyone refuse the water of baptism to these
people, now they have received the Holy Spirit....?’” (Acts 10:46-47).
IS BAPTISM OF DESIRE A SACRAMENT? Baptism of desire is not a sacrament;
it does not imprint the baptismal character or enable a person to receive
the other sacraments. Nevertheless, it does confer sanctifying grace.
https://www.ewtn.com/library/DOCTRINE/BAPTISM.TXT
These are good quotes as well. I especially like the “disciple of the Devil” comment and the clear teaching that the Koran was not revealed by God.