Its a very difficult question. How much do you copy each week? There are fair uses of copyrighted material. My understanding is that anyone can copy, as a reference and as long as credit is given, small portions of a copyrighted material. We see this all the time in books of all sorts copying a paragraph from a reference material.
I would guess if you are systematically copying sections of the CCC as a weekly practice , then you are violating the copyright. But it might be simple to change your format and all would be legal and ethical. For example, start out by just saying what section you are reviewing, providing a link to the online version. Then, throughout your lesson, just copying important paragraphs or sentences that you want to stress: making it clear via formatting that you are copying it and providing correct reference citation. You would likely be just fine at that point.
Here is the relevant law in the US (it is easy to see the intent of the law, but it can get hazy when reading the details:
§ 107 . Limitations on exclusive rights: Fair use40
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
edited to add:
I disagree with the suggestion to contact the UCCCB for permission to copy. It will likely be denied, and you simply have no reason to need to copy the catechism. Also, the denial may be of the form that is somewhat over-the-top from a fair use standpoint (this happens with other types of IP, I know). Then you will be in a quandary of deciding if you should either be using the product fairly.
Finally: do your own research, just use this forum as a means of being pointed in the right direction.