Acts 5:29
We must obey God rather than men.
Abortion and euthanasia are " crimes which no human law can claim to legitimize. There is no obligation in conscience to obey such laws; instead there is a grave and clear obligation to oppose them by conscientious objection. From the very beginnings of the Church, the apostolic preaching reminded Christians of their duty to obey legitimately constituted public authorities (cf. Rom 13:1-7; 1 Pet 2:13-14), but at the same time it firmly warned that “we must obey God rather than men” (Acts 5:29). In the Old Testament, precisely in regard to threats against life, we find a significant example of resistance to the unjust command of those in authority. After Pharaoh ordered the killing of all newborn males, the Hebrew midwives refused. “They did not do as the king of Egypt commanded them, but let the male children live” (Ex 1:17). But the ultimate reason for their action should be noted: “the midwives feared God” (ibid. ). It is precisely from obedience to God"to whom alone is due that fear which is acknowledgment of his absolute sovereignty"that the strength and the courage to resist unjust human laws are born. It is the strength and the courage of those prepared even to be imprisoned or put to the sword, in the certainty that this is what makes for “the endurance and faith of the saints” (Rev 13:10).
In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to “take part in a propaganda campaign in favour of such a law, or vote for it”.
A particular problem of conscience can arise in cases where a legislative vote would be decisive for the passage of a more restrictive law, aimed at limiting the number of authorized abortions, in place of a more permissive law already passed or ready to be voted on. Such cases are not infrequent. It is a fact that while in some parts of the world there continue to be campaigns to introduce laws favouring abortion, often supported by powerful international organizations, in other nations–particularly those which have already experienced the bitter fruits of such permissive legislation–there are growing signs of a rethinking in this matter. In a case like the one just mentioned, when it is not possible to overturn or completely abrogate a pro-abortion law, an elected official, whose absolute personal opposition to procured abortion was well known, could licitly support proposals aimed at limiting the harm done by such a law and at lessening its negative consequences at the level of general opinion and public morality. This does not in fact represent an illicit cooperation with an unjust law, but rather a legitimate and proper attempt to limit its evil aspects. (Pope John Paul II, Evangelium Vitae)