Recently, a question was posed on a social media site which I thought warranted a proper answer. This question was specifically about whether it is licit or illicit (in a Roman Catholic context) to engage in the practices of other religions so long as they do not conflict with Catholicism. The example used by the questioner was Buddhism, a religion I have engaged with extensively by studying Buddhist history, reading the Pali canon, reading Zen masters, and practicing Buddhist meditation.
The question as posed is fundamentally a legal question that should be answered by a study of canon law. I don't recall any prohibitions on engaging in non-Catholic religious practices that are not in contradiction to Catholicism, but it's been many years since I read the Code of Canon Law, so I may have forgotten something.
That said, I don't think that the legal question is the question is the best question to ask. The Catholic faith is not about what's licit, but about what is the most good, the most true, and the most beautiful. Laws may be necessary, but they are far from sufficient. And so let's consider whether it is the most good thing to engage in the practices of other religions where they are not in contradiction with the Catholic faith.
Is it the most good thing to engage in the practices of other religions?
If those other religious practices are in contradiction with the Catholic faith, then a faithful Catholic has an obvious obligation to refrain from participating in those practices. I'm sure that's why the questioner added that caveat up front, and so let's consider practices which are not in contradiction with the faith of the Church.
I have found considerable value in engaging in the practices of other religions, whether I was practicing Buddhist meditation or praying with Muslims. It is very difficult to understand our brothers and sisters in the various religious traditions without experiencing their religious practices, and understanding other religions is indeed a good thing.
St. Thomas Aquinas, a great Doctor of the Church, was quite serious about understanding other religions and having fruitful dialogue with them, though perhaps not the dialogue of contemporary society in which a person would never be so outlandish as to insist that their beliefs were more true than others. He was quite willing to be so outlandish.
And Thomas Aquinas was not making the error so common today of believing that other religions didn't make truth claims which were mutually exclusive with the truth claims of Christianity. He did not suggest that Buddhism is totally, like, a philosophy rather than a religion, man. Nor would anyone who understands Buddhism deeply and sees it as a beautiful whole religion rather than seeing it as a set of practices to be added to the cafeteria tray of spirituality writ large and vacuous.
Buddhism, despite what lots of Westerners might tell you, is a fully fledged religion which is not particularly compatible with the Catholic faith. It is not atheistic; it has an infancy narrative, an eschatology, and makes the claim that Buddha (and Buddhas in general) are divine beings reincarnated to this plane from a heavenly plane, specifically the Tusita heaven. It is not possible to be authentically a Christian while authentically practicing Buddhism, though it may be possible to pick up one or two minor practices from Buddhism which are not at odds with Christianity and do no harm to Buddhism.
There are many good things about Buddhism. Its asceticism, devotional practices, profound understanding of the human condition, and contemplative practices are among them. And the Buddhist contemplative practices do have slightly different benefits than Christian contemplative practices, so a Catholic might get something useful out of it.
In my experience, there is very little to be gained from Buddhist contemplative practices that cannot also be gained in greater abundance from Christian contemplative practices, and in practice the Catholic is better off sticking with practicing the Rosary as a form of contemplative prayer. It is, on the whole, better than Buddhist meditation in my view. And I say that as someone who can only get rid of hiccups effectively by Zen meditation, so I'm firmly in favor of meditation.
I cannot argue that it could never be appropriate to engage in the religious practices of other religions when those practices are not in contradiction with the Catholic faith. At the same time, I would suggest being very careful about engaging in those practices, both out of respect for the integrity of the other religions (avoiding being a cafeteria Buddhist or Muslim in the same way we ought to avoid being cafeteria Catholics) and out of a deep love for the Church, she whose love shows us the way to Christ's love.
If we love the Christ and His Church above all else (as we should), then it makes the most sense to deepen that relationship first. If, as Catholics, we believe that the Church has the fullness of truth (as we should) and that the Church in her wisdom has prescribed for us the best forms of worship and prayer here on Earth (as we should), then it is difficult indeed to make the argument that the most good thing we could do is to engage in the practices of other religions.
And my experience is that the Church has the most good practices for living the spiritual life, so I recommend to any Catholic that they go deep into the practices of the Catholic Church if they are looking for the most good, most true, and most beautiful thing to do.
Note: The above is a picture I took of a Buddhist mālā in my possession.