It’s not a sin not to believe stuff.
The sin would be to will yourself not to believe stuff when you know it’s true.
If you already believe that Jesus set up the Church and revealed to her everything needed for salvation, and set her to teach that to everybody, obviously there are some things you have to will yourself to believe even if you don’t understand how they work yet; or you have to will yourself to disbelieve the whole thing just to stay logical. On the other hand, it’s perfectly workable to say, “Well, I don’t get this, but I believe the Church in general so I’ll trust her on this.”
Beyond that, obviously there’s a big difference in the seriousness of not believing dogma and other basic Catholic teachings about faith and morals, versus disbelieving a particular expression and wording, or a particular section about a subject where the Church does not claim special teaching powers.
Of course, it’s hard to know this difference unless you know a fair amount about Catholicism already.
So I agree that you should ask an apologist about your specific area of concern with the Catechism, or let us know so we can chew it out together.
Re: your original question - In Europe, there did use to be short Catholic question-and-answer catechisms which kids would memorize before their First Communions. You hear about these in stories about saints’ lives, for example, and I’ve seen one that was in Irish, in an old book. I believe these were connected to the Short Catechism that came out from St. Robert Bellarmine.
The Baltimore Catechisms were an old series of Catholic religion primers, designed to be used one per grade level. (They were hardbacks bound in red. A very red sort of red.) Pretty much everybody had them either in Catholic school, or for weekly religion class at the parish if you attended public school. Until the 1960’s, the usual US practice was to teach kids to memorize the short answers to basic questions contained in it.
There were also parts the kids didn’t memorize, which contained instructional material related to the questions, and story questions applying the instructional material to real life. These seem kinda wacky – until you realize they’re written in the same style as canon law case studies, which traditionally had weird fictional names attached to protect privacy.
So you’d have questions for the kiddies like, “Fortunatus doesn’t want to get up on Sunday and go to church. He says God would want him to get more sleep. How would you explain to him the seriousness of missing Mass? What are some ways Fortunatus can get more sleep on Saturday night?” (I just made that one up, but it was like that.)
I just mention this, because it’s something that comes up in US Catholic culture.
If you’re interested, a lot of old Baltimore Catechism memorization questions are online. You will often find: “Why did God make you?”, because that was one of the earliest and most important ones. The answer was: “God made me to know Him, to love Him, and to serve Him in this world, and to be happy with Him forever in the next.”
