Over the last two thousand years there have been different legal, biological, and theological views about the human embryo. Nevertheless, at least within the Catholic tradition, there has been a constant answer to these fundamental moral questions: Yes, the human embryo is sacrosanct. Yes, the human embryo is morally inviolable. Yes, it is a sin to deliberately destroy a human embryo.
In the words of Pope John Paul II: 'Human life is sacred and inviolable at every moment of its existence, including the initial phase which precedes birth'.
In the same way the Second Vatican Council, which is the most authoritative modern statement of Catholic belief, affirms that 'from the moment of conception, life must be guarded with the greatest care'.
The present mainstream Catholic view was the common Christian view until the mid-twentieth century and is still held by most Eastern Orthodox and most Evangelical protestant Christians. It contrasts sharply with ancient pagan belief where infanticide, especially of girls and the disabled, was accepted practice, as was abortion.
The early Christian view of the embryo was similar to ancient Jewish views, but showed an even greater degree of moral concern. I think this was partly because of the place of the infant Jesus in Christian piety. There is a devotion to Jesus not only as a man but as a baby, and there is a devotion to Mary the mother of Jesus not only when she followed him to the foot of the cross but even when she was pregnant with him. There is a famous scene when the newly pregnant Mary meets her cousin Elisabeth who is then six-months pregnant. When Elisabeth sees Mary, her own unborn child leaps for joy. This story occurs at the beginning of the Gospel of Luke. It reminds Christians that Jesus was once an embryo. What is done to human embryos is done to Jesus. That is part of the reason for the protection of the unborn that is characteristic of Catholic Christianity. It is not the whole story but it is part of the story.
It is only if you appreciate just how deep and strong and common is this protective instinct toward the human embryo that you can put into perspective the variations in Catholic belief over the centuries. Catholics have held different beliefs about when the soul enters the body and about when the embryo is not just human but is fully a man (homo). In the Middle Ages, influenced by the mistaken biology of Aristotle, many Catholic theologians believed that the soul was given when the embryo was 'fully formed', at around 40 days or so after conception. However, these theologians did not permit abortion before ensoulment, except in cases of danger to the mother's life. The embryo, even before ensoulment, was already a unique living human creature specially loved by God and being prepared by God to receive a spiritual soul. The embryo before ensoulment was a work of God in progress, a mystery, a human creature on the way to receiving the crowning gift of life from God. It was not disposable. It was not morally equivalent to a nonhuman animal. It was sacrosanct.
These days most Catholic theologians think that the embryo gets a soul from day one and that therefore killing an embryo is homicide. Others believe that it is uncertain when the soul enters the embryo and that to kill an embryo is at least to risk homicide. You may be surprised to learn that the Catholic Church has never officially defined when the soul is given and it is possible to be a Catholic theologian in good standing and hold that the soul is given sometime after conception. What the Church insists on, what has been constant in the Catholic tradition, is not the theological question of ensoulment, it is the moral attitude of respect for human life from conception. The Catholic Church holds and has always held that it is gravely wrong deliberately to destroy a human embryo. Whether or not it is equivalent to homicide, it is certainly a serious act of violence towards a human creature. The human embryo is sacrosanct. You should not do to a human embryo what you would not do to the embryonic Christ.