“Catholicism is a sect/denomination of Christianity, it is not of and by itself singularly Christianity.”
False. The Catholic Church cannot be a sect, nor can it be a “denomination”. Denominations have formed after breaking off from the Catholic Church (e.g. Lutheran groups, Anglican, etc). Christ established ONE Church and that’s the Catholic Church.
If the Roman Catholic Church was “the true” one and only “true Church” in any absolute sense, the other Christian denominations would never have been formed. They were formed because too much of Catholicism was not a testament to intrinsic Christian truth, and too much was just it’s own justification for itself as an institution.