How can we not believe that anymore? How are muslims saved when they reject the trinity?
As an initial point, while kind of a topic on its own, Muslims acknowledge the one God–which can be done without faith–but not the Trinity of Persons, which, as a revealed truth, requires faith to believe.
In any event, the Church still teaches it is impossible to be saved without faith in Christ. But one can have faith in Christ while innocently believing an error about particular articles of faith (since all of revelation is found in Christ the Word, faith in Him embodies the reception of the whole truth; on the flipside, for the same reason culpable heresy destroys all faith). From the Catechism:
The Necessity of Faith
161 Believing in Jesus Christ and in the One who sent him for our salvation is necessary for obtaining that salvation.42 "Since “without faith it is impossible to please [God]” and to attain to the fellowship of his sons, therefore without faith no one has ever attained justification, nor will anyone obtain eternal life ‘But he who endures to the end.’"43
Faith in Christ is necessary because faith “is a free assent to the whole truth that God has revealed.” (CCC 150) and Christ “is the Father’s one, perfect and unsurpassable Word. In him he has said everything; there will be no other word than this one” (CCC 65) and “what he spoke before to the prophets in parts, he has now spoken all at once by giving us the All Who is His Son” (CCC 65, quoting St. John of the Cross).
Even if in good conscience, people in non-Christian religions cannot have faith–they simply do not believe what God has revealed. Their belief is merely “religious experience still in search of the absolute truth and still lacking assent to God who reveals himself” and therefore “the distinction between theological faith and belief in the other religions, must be firmly held.” (Congregation for the Doctrine of the Faith, Dominus Iesus 7).
However, we do acknowledge that, for those in good conscience seeking to follow the truth, “in ways known to himself God can lead those who, through no fault of their own, are ignorant of the Gospel, to that faith without which it is impossible to please him.” (CCC 848).
Likewise, as I mentioned earlier, if one has faith in Christ, but is innocently ignorant or erring on some article of faith, it does not destroy faith. Since faith is one based on the person of Christ, "neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it.” (St. Ireneaus, Against Heresies I, 10, 2). For this reason, St. Augustine says in letter 43:
But though the doctrine which men hold be false and perverse, if they do not maintain it with passionate obstinacy, especially when they have not devised it by the rashness of their own presumption, but have accepted it from parents who had been misguided and had fallen into error, and if they are with anxiety seeking the truth, and are prepared to be set right when they have found it, such men are not to be counted heretics.
