The Christian church is the Catholic Church. Christianity (as the label is used today, not as it is in Acts) is a kind of Catholicism, in imperfect communion. "Catholic Christianity" is "Christianity" proper. Other "Christianity" is Christianity, without the fullness of Truth.
The Catholic Church is not just one denomination amongst many, as many seem to view it.
And, no, I see nothing wrong with reading the fictional scriptures of other religions, as long as you are strong in faith and well-learned and catechized. I don't think anyone has ever been converted to Islam by reading the Koran - it's one of the worst and most boring books ever written, and I consider Aristotle thrilling, although in the original Arabic the cadences of the sentences and the rhyme-structure is engaging; the actual meaning is disjointed and inarticulate, held together by rhyme instead of theology. If you read a Koran, may I recommend either the AJ Arberry translation (the scholarly standard, no notes) or the Noble Qur'an by Muhammad Muhsin Khan (which is the closest one gets to the actual, mainstream orthodox Muslim interpretations and understanding of the meanings of the Koran which is very incomplete by itself - that's why there are dozens of volumes of ahadith, to fill in the gaps and make a coherent religion - and nearly impossible to understand, like breaking in on a private conversation halfway through; the Khan Koran is the one left in hotel-room drawers in the Middle East as the Gideon N/KJVs are in America).
I've read the scriptures of all of the world religions (Judaism, Christianity, Islam, Hinduism, Sikhism, Daoism, Confucianism), and several minor religions (Mormonism, Urantianism, Platonism, Gnosticism, ancient Greek and Roman religion) except for Buddhism (I've read the Dhammapada and a few sutras only: the Buddhist scriptures are 100 large volumes), and Zoroastrianism (the Avesta).
I've read all of the apocrypha too, contained in four large volumes of about 1000 pages each: two for the OT, two for the NT. I imagine for most people reading the supposedly "secret" or "forgotten" or "suppressed" Gospels/writings is even more dangerous, as they're closer to Christianity, if one doesn't have a solid grasp of Church History and the process of Canonical formation, etc. instead of the ******** line fed by many "popular scholars" like Bart Ehrman and Elaine Pagels (and the Jesus Seminar) today, about "heresy preceding orthodoxy" and "multiple Christianities" (they had a word for those other "Christianities" - heresies), and "lost gospels of equal validity to the canonical ones" (even though, without exception, the "lost gospels" are closer to the fantastic and impossible like the Koran and Book of Mormon - the Koran borrows a lot from the apocrypha - and are dependent on the canonical gospels, which were written a minimum of one century earlier (the earliest apocryphal Gospel is that of Thomas, written in the mid-to-late 2nd century AD; the others were written in the 3rd and 4th, far removed from their source and the Apostolic preaching and authority which delineated the canon), and "moving Christianity back to its tolerant, inclusive, pluralistic [supposed] roots, and beyond theism, doctrine, dogma, and morality" - people who've bought those (discredited in the 1970s) things hook, line, and sinker shouldn't be reading the noncanonical writings.
If one struggles with these bogus concepts of liberal/secular/inclusivist/tolerant/atheistical "church history", read Heresy and Orthodoxy: How Modern Culture's Obsession with Diversity is Reshaping our Understanding of Early Christianity by Andreas Kostenberger.