The Catechism of the Catholic Church states: "Since the first century the Church has affirmed the moral evil of every procured abortion. This teaching has not changed and remains unchangeable. Direct abortion, that is to say, abortion willed either as an end or a means, is gravely contrary to the moral law" (No. 2271).
From earliest times, Christians sharply distinguished themselves from surrounding pagan cultures by rejecting abortion and infanticide. The earliest widely used documents of Christian teaching and practice after the New Testament in the 1st and 2nd centuries, the Didache (Teaching of the Twelve Apostles) and Letter of Barnabas, condemned both practices, as did early regional and particular Church councils.
To be sure, knowledge of human embryology was very limited until recent times. Many Christian thinkers accepted the biological theories of their time, based on the writings of Aristotle (4th century BC) and other philosophers. Aristotle assumed a process was needed over time to turn the matter from a woman's womb into a being that could receive a specifically human form or soul. The active formative power for this process was thought to come entirely from the man – the existence of the human ovum (egg), like so much of basic biology, was unknown.
However, such mistaken biological theories never changed the Church's common conviction that abortion is gravely wrong at every stage. At the very least, early abortion was seen as attacking a being with a human destiny, being prepared by God to receive an immortal soul (cf. Jeremiah 1:5: "Before I formed you in the womb, I knew you").
In the 5th century AD this rejection of abortion at every stage was affirmed by the great bishop-theologian St. Augustine. He knew of theories about the human soul not being present until some weeks into pregnancy. Because he used the Greek Septuagint translation of the Old Testament, he also thought the ancient Israelites had imposed a more severe penalty for accidentally causing a miscarriage if the fetus was "fully formed" (Exodus 21: 22-23), language not found in any known Hebrew version of this passage. But he also held that human knowledge of biology was very limited, and he wisely warned against misusing such theories to risk committing homicide. He added that God has the power to make up all human deficiencies or lack of development in the Resurrection, so we cannot assume that the earliest aborted children will be excluded from enjoying eternal life with God.
In the 13th century, St. Thomas Aquinas made extensive use of Aristotle's thought, including his theory that the rational human soul is not present in the first few weeks of pregnancy. But he also rejected abortion as gravely wrong at every stage, observing that it is a sin "against nature" to reject God's gift of a new life.
During these centuries, theories derived from Aristotle and others influenced the grading of penalties for abortion in Church law. Some canonical penalties were more severe for a direct abortion after the stage when the human soul was thought to be present. However, abortion at all stages continued to be seen as a grave moral evil.
From the 13th to 19th centuries, some theologians speculated about rare and difficult cases where they thought an abortion before "formation" or "ensoulment" might be morally justified. But these theories were discussed and then always rejected, as the Church refined and reaffirmed its understanding of abortion as an intrinsically evil act that can never be morally right.
In 1827, with the discovery of the human ovum, the mistaken biology of Aristotle was discredited. Scientists increasingly understood that the union of sperm and egg at conception produces a new living being that is distinct from both mother and father. Modern genetics demonstrated that this individual is, at the outset, distinctively human, with the inherent and active potential to mature into a human fetus, infant, child and adult. From 1869 onward the obsolete distinction between the "ensouled" and "unensouled" fetus was permanently removed from canon law on abortion.
Secular laws against abortion were being reformed at the same time and in the same way, based on secular medical experts' realization that "no other doctrine appears to be consonant with reason or physiology but that which admits the embryo to possess vitality from the very moment of conception" (American Medical Association, Report on Criminal Abortion, 1871).
Thus modern science has not changed the Church's constant teaching against abortion, but has underscored how important and reasonable it is, by confirming that the life of each individual of the human species begins with the earliest embryo.
Given the scientific fact that a human life begins at conception, the only moral norm needed to understand the Church's opposition to abortion is the principle that each and every human life has inherent dignity, and thus must be treated with the respect due to a human person. This is the foundation for the Church's social doctrine, including its teachings on war, the use of capital punishment, euthanasia, health care, poverty and immigration. Conversely, to claim that some live human beings do not deserve respect or should not be treated as "persons" (based on changeable factors such as age, condition, location, or lack of mental or physical abilities) is to deny the very idea of inherent human rights. Such a claim undermines respect for the lives of many vulnerable people before and after birth.