The respect for human life and the responsibility to protect the innocent, is firmly centered within the Catholic faith, and upon it rests the mandate against abortion and euthanasia, and the support for just war and capital punishment.
The Church was largely alone in the mandate against abortion in the ancient world, as this post by Chiesa, responding to the publication of a book about abortion within the current nation of China.
There are many within the Church and without who have fought against these ancient teachings of Catholicism, and though the teachings often appear on shaky grounds, they have held, and the constancy of each—as reflected within two central documents of the magisterium, the Catechism from the Council of Trent and the Catechism of the Catholic Church from the Council of Vatican II—is crucial for the Church.
An excerpt from the article from Chiesa.
“One of the ideas that recur most in the writings of the first Christians is in fact their desire to frequently repeat one concept: we Christians are different from the pagans, in part because we do not kill our children, neither within our women's wombs or outside of them.
“In chapter XXX, paragraph 2 of his "Octavius," the second-century apologist Minucius Felix, comparing the teaching of Christ with that of the pagans, writes: "you expose your newborn children to wild beasts and to birds; or strangling them you crush with a miserable kind of death. There are some women who, by drinking medical preparations, smother in their very bowels the seed destined to become a human creature, and thus commit a parricide before they bring forth. And you learn these things from your gods, for Saturn did not simply expose his children, but even devoured them."
“For his part, the great Tertullian, in his "Apologeticum," chapter IX, states: "For us Christians murder is expressly forbidden, and therefore it is not even permitted for us to destroy the fetus in its mother's womb. Preventing birth is murder in advance. It doesn't matter at all whether one destroys a life already born or crushes it at birth: what is about to be born is already a human being. Every fruit is already contained in its seed."
“Another very important document from second-century Christianity, written in Asia Minor, the Letter to Diognetus, reiterates the same ideals in this rather concise manner: "Christians marry like everyone else and produce children, but they do not throw away their newborns."
“On this same theme of infanticide, the historian A. Baudrillart has written: "There may be no matter on which ancient pagan society and modern Christian society are in more stark opposition than in their respective ways of thinking about children."