You are confusing what you learned in Chatechism class with unchangeable truth. The essetial truth, according to St. Paul, is that there were witnesses to the risen Christ and that we are their heirs. We can always debate sexual ethics and dogma about the Triity is fixed because we have all agreed to it in our baptismal promises and Creed and because, aside from agreement, there is no way of knowing the truth of them in this life. The Mass is entirely changeable. We know that because it has, in fact, changed.