In order for a "Muslim" to be saved, he would have to be a good Catholic, which is to say that he would not be a formal member of Islam at all, but only a material one.
But that is no different from heretical Rahnerian Anonymous Christianity - your Muslim has no faith in Christ nor Trinity and is supposedly a Catholic without knowing it. If it is possible for a Muslim, so it must be for a Jew, Animist, Buddhist etc. What you eventually end up with is that anyone can be saved in any religion, which reduces EENS to nothing.
There were Arians and there were semi-Arians. There were Pelagians and there were semi-Pelagians. St. J the E is not a liberal, he is a semi-liberal.
Actually, I'm perfectly capable of accepting everything St. Augustine said, and believing in strict visible unity with the Catholic Church & water Baptism are absolutely necessary for salvation. It's just that I have to reconcile this view with the views of all the theologians of the last few centuries that have taught baptism of desire, often even implicit desire. These weren't all liberals either. I hope for greater clarity on EENS from the Magisterium.
In the history of the Church many people have presumed to attempt to reconcile orthodoxy with heresy, the result being all of the "semi" heresies, the semi-Pelagians, semi-Arians, etc.
Maybe the cause of your uncertainty is that you are seeking to base your faith on human testimony instead of Divine tesimony. God can neither deceive nor be deceived. Humans can be deceived and can deceive others. When Queen Isabella brought a moral question to her confessor, he began to quote certain of the Doctors and esteemed moralists of that day. "St. Augustine said...; St. Gregory said...; St. Thomas said..." The saintly queen interrupted him and said: "Father, I do not want to know what the Fathers said, good as they were; I want to know what the Church says." Fr. Wathen related this story about Queen Isabella in
Who Shall Ascend and it contains a good lesson for all Catholics.
Ex Cathedra: “The most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the eternal fire which was prepared for the devil and his angels, unless before death they are joined with Her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgivings, their other works of Christian piety and the duties of a Christian soldier. No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved, unless he remain within the bosom and the unity of the Catholic Church.” (Pope Eugene IV, the Bull Cantate Domino, 1441.)
Vatican I: “
We teach and define as a divinely revealed dogma that when the Roman pontiff speaks
EX CATHEDRA, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole church, he possesses, by the divine assistance promised to him in blessed Peter, that
infallibility which the divine Redeemer willed his church to enjoy in defining doctrine concerning faith or morals.
Therefore, such definitions of the Roman pontiff are of themselves, and not by the consent of the church,
irreformable. So then, should anyone, which God forbid, have the temerity to reject this definition of ours: let him be anathema.”
Vatican I: ''The doctrine of the faith which God has revealed has not been proposed to human intelligences to be perfected by them as if it were a philosophical system, but as a divine deposit entrusted to the Spouse of Christ to be faithfully guarded and infallibly interpreted.
Hence also that sense of the sacred dogmas is to be perpetually retained which our Holy Mother the Church has once declared, nor is this sense ever to be abandoned on plea or pretext of a more profound comprehension of the truth.”
Pope St. Pius X, The Oath Against Modernism: “I sincerely hold that the doctrine of faith was handed down to us from the apostles through the orthodox Fathers in exactly the same meaning and always in the same purport.
Therefore, I entirely reject the heretical' misrepresentation that dogmas evolve and change from one meaning to another different from the one which the Church held previously.”
Saying that non-Catholics can be "anonymous Christians" and saved without converting is an example of an "evolution of dogma." It is heretical, and you are sorely mistaken if you believe that St. John the beloved or any of the other Apostles or Evangelists believed such a doctrine.