Since the official, established, and formal “teachings of the Catholic Church” are TRUE … It CAN be.
Is the questioning HONEST questioning? FAIR questioning? Publically and disrespectfully questioning? Cynical and dismissive questioning?
Or reverent, thoughtful questioning as in Peter’s " … Lord, to whom shall we go? You have the words of eternal life." Or Mary’s " … pondering all these things in her heart."
Questioning of the nature of*** " … how may I understand this better?" or*** " … what are the fruits of this teaching for the faithful?" is quite different from a "why do I have to do THIS?" semi-tantrum.
Asking “why” honestly and fairly with a good attitude toward the Church can lead to greater insights. Questioning in the sense of “this teaching, though true, could be refined and more powerful if it is married to THIS other Church teaching - for the people of our time …” is quite a different thing than egoistic rebellion. And could lead to building UP the Church vs. tearing her down.
If your own questioning is a prompting of the Holy Spirit … of course no, it’s not a sin.
Such questions will lead one to deeper truth, spur the use of the Holy Spirit’s gifts within us (Knowledge, Understanding, Wisdom, Counsel, Piety, Fortitude and Fear of the Lord) and lead us to answers that will enlighten others - and leading toward the joyful “finding” of the Lord’s “…seek and you shall find” counsel.
The Lord also told the apostles “who hears you hears me …” and sent them forth, deputized as it were with many powers, including the teaching power “Go … and teach all nations …”
The apostles then had, and the Church of today has - “teachings”. Some formal and defined by Councils or a Pope’s “ordinary teaching”, also lesser rulings (temporary or permanent). On occasion there were (and are) controversies undefined by the Church “teachings;” some with clergy on either “side” for a time. Which clergy do I follow? Shouldn’t I question ‘which is right?’
Could BOTH be right … but misunderstanding one another? Both right in that one Bishop was granting a local dispensation (say) while another broadly defending a teaching of the Church? Is one “Churchman” or “Sister” challenging established dogma and the other defending it?
Well … these are questions, but not sinful if asked in sincerity and with an honest desire to please the Lord rather than indulge our feelings.
On a “put on the armor …” note, Satan will try to lead us to “question the teachings …” in the worst possible way. Just as he did with Eve and the Lord’s teaching “do not eat the fruit of the tree in the middle of the garden … for on the day you eat of it you shall die.”
“Did the Lord really say THAT?” was Satan’s opening gambit … “questioning the teaching” if you will. Soon the question became a list of accusations “You will not DIE. He knows you will become like HIM, like gods, knowing right from wrong …” and Eve, guided by such questions and declarations, questioned FURTHER. But justifying each contradiction of God’s “teaching” with an interior “why NOT?”
Is the fruit desirable? Yes it looks delicious!
Would we be more like gods if this knowledge is in this fruit? Yes, that’d be desirable too.
Wouldn’t eating it help me find out who’s telling the truth (God or this snake)? YES!
If it is as the snake says, wouldn’t God just be selfish and bossy, a liar and in the way of our self-actualization? Yeah. Him and His special stash!
And the “questions” led to rebellion and ruin. And were “sinful questions” before she even sinned (considering their source). Questioning wasn’t the sin, eating the fruit was.
But before the eating came doubt, suspicion, envy, pride, moral equivalence, and coveting another’s goods.
Conclusion: Given the*** “Is it a sin?”*** question above and the Eve example … a ringing **YES and a childlike obedience that doesn’t question so much – might be more prudent **than all the mental gymnastics gone through above.
The **NO column is fraught with much more danger **… even if it can be argued that "holy questioning leads to greater understanding, knowledge, and wisdom…" etc.
******If we do question, invite the Holy Spirit in on it … instead of letting Satan “raise our consciousness” in a spirit of selfish “protest”.