First, here's what Pope John Paul II authoritatively wrote:
....we need now more than ever to have the courage to look the truth in the eye and to call things by their proper name, without yielding to convenient compromises or to the temptation of self-deception. In this regard the reproach of the Prophet is extremely straightforward: "Woe to those who call evil good and good evil, who put darkness for light and light for darkness" (Is 5:20). Especially in the case of abortion there is a widespread use of ambiguous terminology, such as "interruption of pregnancy", which tends to hide abortion's true nature and to attenuate its seriousness in public opinion. Perhaps this linguistic phenomenon is itself a symptom of an uneasiness of conscience. But no word has the power to change the reality of things: procured abortion is the deliberate and direct killing, by whatever means it is carried out, of a human being in the initial phase of his or her existence, extending from conception to birth. The moral gravity of procured abortion is apparent in all its truth if we recognize that we are dealing with murder and, in particular, when we consider the specific elements involved. The one eliminated is a human being at the very beginning of life....That's from Evangelium Vitae 58 (; emphasis added and needed). It should be enough to rebut the assertion I began this post by quoting. But the way liberal theologians try to get around the rebuttal is to make the following argument:
(1) No particular opinion as to the moment of the conceived child's "ensoulment" is part of the definitive teaching of the Church.
(2) Ergo, granted that a human embryo is a human being, it does not follow from the definitive teaching of the Church that every such being is a human person and thus a subject of inherent human rights, such as the right to life.
(3) Ergo, the Church's definitive teaching is not that any and every procured abortion is a homicide and thus morally wrong, but at most that we should treat them as gravely as homicide in order to be "on the safe side."
Now (3) is one thesis of that venerable theory in moral theology which is known as "tutiorism": when in doubt, do the safer (tutior) thing. The standard liberal view is, accordingly, that what's wrong with abortion is not that we know it's murder; rather, given that we don't know it isn't murder, we shouldn't risk it and we do so only by diminishing our respect for life. But as already indicated, the Pope's view is stronger than that. Some theologians consider it "extremist." Perhaps so; but it no more extreme than Extreme Truth Himself. For the reasons why John Paul II took the view he did also show what's wrong with the tutiorist view.
Its vision is blinkered because, once it be granted that a human embryo is a human being—not merely a part of a human being such as a gamete or an organ—it would be incompatible not only with the definitive teaching of the Church, but also with sound philosophy and modern natural science, to deny that every human embryo is a human person with the inherent dignity and rights of human persons, starting with the right to life.
Let's start with the Catechism of the Catholic Church:
365 The unity of soul and body is so profound that one has to consider the soul to be the "form" of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.
From that, it follows that whatever is a living, human body is a human being with a single human nature characterized by a profound unity of body and soul. Clearly the human embryo—and for that matter, the human zygote—is such a living human body, albeit at early, relatively unarticulated stages of development. Therefore, if the above-quoted teaching of the CCC is true—and it is the definitive teaching of the Church—then anything counting as a living human body is a human being with a soul every bit as spiritual as yours and mine.
Now, to insist nonetheless that some such an entity might fail to qualify as a human person, with all the inherent rights thereof, requires assuming that personhood is not a kind of being but rather a functional state. In other words, what makes a human being a person, not just a biologically human life form, is not that it is an individual substance with a spiritual soul, but rather that it has attained a stage of development in which its natural functioning is characteristic of persons as such—chiefly, rational thought and free will. One is thus a person not in virtue of what one is but in virtue of what one is in a position to do—or at least in virtue of what others are willing to count as sufficient potential. Yet the moral consequences of such a view are utterly chilling, even aside from its obvious incompatibility with the teaching of the Church. I doubt I need to explain why; at least, I don't know any Catholics prepared to come right out and defend, like certain secularist philosophers, the purely functionalist view of personhood. If they are, then Mein Kampf would be better suited to their worldview than the New Testament.
It would seem, therefore, that the only way for Catholics to steer between the Pope's view on the one hand and heresy on the other is simply to deny that we can know whether each and every normally developing human blastocyst/zygote/embryo/fetus is a living human being. Well, one can certainly avoid the Pope's view that way; but avoiding heresy as well comes only at the price of denying what natural science has made obvious since the 19th century. Rather than belabor the point here, I refer readers to the relevant passage and context of Prof. Robert George's excellent article "God's Reasons".
Tutiorism is therefore outdated as well as blinkered. Given the advances in our scientific knowledge, it is too weak philosophically, theologically, and morally. The late pope's view shows that the Church is moving beyond it. True, it was much more plausible at a time in intellectual history when natural science really didn't have much to say about prenatal development beyond what could be seen with the naked eye. It was actually the standard view among Catholic theologians for that time; and even today, tutiorism might be safer than some of the alternatives for non-Catholics. But for informed, honest Catholics today, it is neither "safe" nor worthy.