The Church expects a humble submission to that which has been established and held from the beginning (with development of doctrine and increased understanding along the way).
We call these doctrines that all Catholics must accept de fide dogmas (or in some cases, ex cathedra). In a nutshell, though there are fine (sometimes very fine) distinctions that can be drawn, the Catholic is obliged, in the nature of the case, to accept all that the Church teaches.
It's an act of faith to accept the notion that the Catholic Church is the One True Church established by Jesus Christ, historically continuous, universal, specially protected by the Holy Spirit for the purpose of passing down the apostolic deposit.
Because we accept this in faith as Catholics, it is required that we accept all that the Church teaches. To do that is itself a result of supernatural grace from God.
Protestants don't look at it that way because, first of all, they deny that the Church is infallible. Once one does that, then it is a completely new rule of faith. For Protestants, this rule is sola Scriptura, or the idea that Scripture (excluding an infallible Church and tradition) is the only infallible authority.
Therefore, it is very difficult for a Protestant to accept the notion of submitting to a Church's teaching in faith, because for them, no Church has it completely right: only the Bible does that. They often see this as virtually a violation of people's right to think for themselves, or contrary to one's conscience.
It's not at all: it is a recognition of our own limitations, of something higher than ourselves: established by God, and of a Christian faith that includes acknowledgment of an authoritative, infallible Church. It takes faith, and faith is a supernatural gift granted by God through His grace and the assistance of the Holy Spirit. It's not mere reason, but can be supported by reason: like all Christian dogma.
Some Protestants use the word dogma, but they generally prefer the word doctrine. Protestants, to the extent that they are serious about historic Christian doctrine, do, of course, require a set of beliefs, too: just not as many as we do. I always use the example of Calvinists. A truly Calvinist denomination (Presbyterian, Reformed, in their traditional forms and belief-structures) would not allow a member to deny all five tenets of TULIP:
Perseverance of the Saints
Even disbelief in two or three would not be permitted. They would no longer be considered members of the denomination, in terms of adherence to the creed of the group.
There are always, it's true, people who join groups but disagree on some of the tenets. Catholics are not literally disallowed to have doubts and uncertainties in their head (the Church is not simplistic or naive about such things), but they are supposed to accept in faith all that the Church teaches, and certainly not openly oppose it, whether they fully understand everything or not.
As an analogy from my Protestant days, I attended Assemblies of God for four years (1982-1986). I never agreed with one of the 16 Fundamental Truths: that everyone in the denomination were required to adhere to: the one (actually a combination of #7 and #8) asserting that everyone had to speak in tongues in order to receive the “enduement of power.”
This I felt to be expressly contrary to Paul's teaching about tongues, where he says that not all speak in tongues (1 Cor 12:30). Because of this, I never became a member. I was simply being honest: saying in effect: “I don't accept all 16 Truths that I am supposed to accept, so I won't become a member, but I agree with most everything else taught.”
So what do we do when we disagree with something in the church we attend? If we say we are Catholics, then it should be understood that we accept all Church teaching, and are not reserving the right to express private judgment in dissent against the Church.
To say that one is a member of the Assemblies of God is to accept the 16 Fundamental Truths, which is pretty much its confession. I did not, so I didn't become a member. I wasn’t trying to be controversial; I was being true to myself, and, I think (then and now) to St. Paul's teaching, that I believe contradicts Assembly of God teaching.
The Catholic Church doesn't expect everyone to have perfect faith or knowledge and to fully understand every jot and tittle. But the Church expects a humble submission to that which has been established and held from the beginning (with development of doctrine and increased understanding along the way).
There are always, despite all these considerations, people who are “in” a group but not totally “of” it. Many times, they don't properly understand the teachings of their own group. Other times, they do fully understand, and disagree, as I did in the Assemblies of God.
I did what I felt was the only honest thing, and did not ever become a member, because that would have entailed ostensible agreement with a tenet that I did not accept. It would have been dishonest. One must understand what a group officially teaches.
To make this concept of “accepting all that the Church teaches” fairly simple and practical, Catholics should be expected to agree with, for example, what is included in the Catechism of the Catholic Church. It was promulgated under Pope St. John Paul II in 1992, accompanied by the Apostolic Constitution Fidei Depositum, which stated in part:
The Catechism of the Catholic Church, which I approved 25 June last and the publication of which I today order by virtue of my Apostolic Authority, is a statement of the Church's faith and of Catholic doctrine, attested to or illumined by Sacred Scripture, the Apostolic Tradition and the Church's Magisterium. . . .
This catechism is given to them that it may be a sure and authentic reference text for teaching Catholic doctrine and particularly for preparing local catechisms.