St. Alphonsus - “The Mohammedan Paradise, is only fit for beasts; for filthy sensual pleasure is all the believer has to expect there.” (St. Alphonsus de Liguori, History of Heresies, Vol. 1., ch. vii., art. 1.)
Baltimore Catechism (No. 3) states as follows:
Q. 1148. How do we offer God false worship?
A. We offer God false worship by rejecting the religion He has instituted and following one pleasing to ourselves, with a form of worship He has never authorized, approved or sanctioned.
St. Peter Canisius “Who is to be called a Christian? He who confesses the doctrine of Christ and His Church. Hence, he is truly a Christian thoroughly condemns and detests, the Jewish, Mohammedan, and the heretical cults and sects.” (Catholic Cate-chism, Dillingen, 1560,)
St. Peter Mavimenus -, “We worship the same God, all is well” No! He told them the truth, he put it this way to them “Whoever does not embrace the Catholic Christian religion will be damned, as was your false prophet Mohammed.” (The Roman Martyrology for February 21)
St. Augustine, - letter to St. Jerome “one who observes the rites of Jews, or Gentiles, not only truly, but even fictitiously, has fallen into the abyss of the devil.”
Suarez - “he, who unites himself with those outside the Church in religious worship, communicates in their wicked works.”
(Opera Omnia, Tom. XXIV, p. 708)
Fr. Michael Müller, C.SS.R - “A Mohammedan is taught by his conscience that it would be a crime to believe in Jesus Christ, and not believe in Mahomet; will this impious conscience save him? The Scripture assures us that ‘there is no other name given to men under heaven by which we can be saved,’ but the name of Jesus only; and 'he that believeth not the Son shall not see life, but the wrath of God remaineth on him.”
“All the Gods of the Gentiles are Devils” (Psalm 96:5)
St. Thomas - “Nor can anyone in any way know God who holds a false opinion of Him” (Summa Theologica II - II q. 10, Art. 3)
St. Francis of Assisi - "when reproached by the followers of Koran for preaching against Mohammed, simply responded by saying “We have come to preach faith in Jesus Christ to you, that you will renounce Mohammad, that wicked slave of the devil, and obtain everlasting life like us” (Omer Englebert, 1979, Pg. 178-9)
St. John Vianney - “My friend, there are not two ways of serving Our Lord; there is only one good way, and it is to serve Him as He wishes to be served”. (SPIRIT OF THE CURE D’ARS, Bowden, 1864)
St. Alphonsus - “But the holy monk (St. George of San Saba) having declared that Mahomet was a disciple of the devil, and that his followers were in a state of perdition, he also was condemned (to martyrdom) with his companions.” (Victories of the Martyrs, ch. LIII.)
Blessed Nicholas Tavilich - “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.”
St. John- “If anyone comes to you and does not bring this doctrine, do not receive him into the house, or say to him Welcome” - II John 1:10