Can Catholics morally object to sharia law?

Clarifying the question

Although I understand the intent of the question (which I think is interpretation #1 below), I just want to point out there are at least 5 possible ways that the Q can be interpreted. I am also going to answer #5. The answers are based on the religious authority of the Catholic church because a Catholic needs to obey his/her religion FIRST when there is a conflict with one's obligation to the state/country.

1. Can a Catholic refuse to obey an existing law based on Islamic Sharia? Answer in the other section below.
2. Can a Catholic participate in a political protest to reduce the punishment for breaking an existing Sharia-inspired law? Yes, unless prohibited by the state / Sharia.
3. Can a Catholic participate in a political protest to de-criminalize an existing Sharia-inspired law? Yes, but only when the law is against NT Divine Law or against Natural Law, AND if the protest is permitted by the state / Sharia.
4. Can a Catholic participate in a political protest to prevent a proposed implementation of a Sharia law in his/her state? Yes, unless prohibited by the state / Sharia, but only when the Sharia law is considered a Human Law.
5. Can a Catholic lawmaker or a sovereign promotes implementing Sharia in his/her state / country, or lobbies for the implementation of a separate religious court for certain areas of human life? Answer in the other section below.

Also, the question does not state the political and religious situation of the state/country in which the Catholic resides. I am going to assume scenario #1 below:

1. The Catholic is a resident of a Muslim majority nation that implements Sharia law for everyone, not just for Muslims (example: Iran).
2. The Catholic is a resident of a Muslim majority nation that implements European style law in most cases but supplemented with Sharia court (example: Malaysia). Two types of cases:
   • cases where one of the parties in dispute is Muslim (example: contract dispute) so the case is tried in a Sharia court, or Sharia law is used in the general court
   • cases where general civil & criminal law has been influenced by Sharia law (let's say certain physical intimacy in public are considered crime even when the couple are not Muslims)
3. The Catholic is a resident of a Muslim majority nation (example: Indonesia) but residing in a province (example: North Sumatra) where there is intense discussion to follow a neighboring province which already implements Sharia law fully (example: Aceh).

Background: Catholic Morality

The foundation of morality for a Catholic is: freedom to love according to Catholic understanding of love. St. Augustine famously said: "Love and do what you will".

In Catholic perspective, informed by Aquinas, laws for this love+freedom-based morality has a hierarchy (see Aquinas On Law):

1. Eternal law : Laws based on God's own reason (unknown to us), which inform Divine and Natural law.
2. Divine law : Revealed laws in the OT and the NT.
3. Natural law : Implanted laws in human conscience, same for everyone regardless of religion since it's part of human nature, the basis of objective morality.
   • Because human nature is corrupted by the Fall, knowing this law is not very reliable, so this knowledge needs to be verified and supplemented by Divine law.
   • In principle, Divine law and Natural law should be in harmony, interpreting each other. For example, Divine Law is revealed through Bible authors which were later committed to the text we have today, while Natural Law is revealed to billions of human souls across culture and ages. By careful cooperative process of reflecting on this Natural Law using philosophy of religion and moral philosophy, we can apply Divine Law (interpreted properly) to new modern practical moral circumstances (such as contraception, gene editing, etc). This is an example of Faith and Reason fusing different areas of competence harmoniously.
4. Human law: Ideally, laws that people create within a government, should not contradict Divine and Natural law which are one level of authority above.
   • Catholic understanding of the purpose of Human Law (based on Aquinas) is for the promotion of virtue necessary for the common good. The kind and the level of punishment should be intimately tied to this goal. Catholic understanding is very different than seeing Human Law as merely an "executor of divine command" which for example authorizes stoning for adultery.
   • Catholics do not see Divine Law as prescribing punishment, but as a guidance to stay living "on the right path" (cf. Ps 119:35: "Direct me in the path of your commands, for there I find delight.") Thus Human Law constructed in the spirit of Divine Law should then be a cause of joy and delight, because it provides a visible boundary of "safe path".
   • Human law takes as guidance the Divine and Natural, adapted to particular geographical, historical, and social circumstances. Thus it makes sense to have Human Law being continually adapted to modern realities, especially where we undergo technological changes and face new economic arrangement (such as global trade, labor unions, women participation in the labor force, etc.).
   • Those in power nowadays are usually good in politics but not in theology or philosophy, which require a religious atmosphere attuned to God and a professional academic environment to apply reason to process enormous amount of research cooperatively. Aquinas had both and the Summa is a product of his piety as well as the early Dominican environment to write it. Thus, Catholic jurisprudence and Christian philosophy of law should then advise those who are in power, whether it is a theocracy, a constitutional monarchy, a republic, a democracy, etc.

Can a Catholic refuse to obey an existing Sharia law?

First the Catholic needs to place the particular Sharia law within the hierarchy above. Muslims put all Sharia laws at level #2 (Divine law), but Catholics need to evaluate on a case by case basis. Catholics can also consider an individual Sharia law as #2 (Divine law) only if it matches an NT law (such as do not murder an innocent). But if a judicial OT law (the topic of Summa Theologica I-II, Question 104) is not applicable in NT, Catholics will consider the Sharia law as #4 (Human law) which can be either consistent with #3 (Natural Law) or not. This is also why the punishment prescribed can be modified, since it is no longer Divine Law.

Example Scenarios:

1. If the law is mandated by Catholic Divine law (NT) or Natural law, the Catholic does NOT have moral ground to refuse.
2. If the law is an obligation that is not mandated by Catholic morality AND it does NOT violate Divine (NT) or Natural Law, the Catholic should obey it for reasons of peace with society. It simply reduces his/her freedom, but not a sin to obey.
3. If obeying the law makes a Catholic sin against Divine (NT) or Natural law, then the Catholic DOES have moral grounds to refuse.
4. When the Sharia law is determined to be in the Human Law category, Catholics are free to use the available political process to influence a change in the name of freedom and love. For example, to campaign for better laws for women.
5. When the Sharia law is determined to be in the Divine Law category (such as do not steal), Catholics can object to unnecessarily harsh punishment.
   • We have to understand that punishment is a separate issue than whether to obey the law. This reflects Western law and court systems where determination of guilt is separated entirely from the determination of punishment (sentencing phase).
   • Since OT judicial laws are no longer in force for Catholics, and since NT laws (which is based on freedom and love) don't prescribe punishments other than excommunication or other church sanctions, church authorities delegate punishment to secular authorities IF the secular authorities find that the person ALSO violates secular law.
   • In the case where the "secular" law is Sharia law enforced by an Islamic theocracy, Catholics are morally free (in the eyes of the Catholic Church) to protest the level of punishment through allowable political process since in the Catholic moral system punishment is not part of Divine Law but of Human Law.
   • CONCLUSION: Even when a Divine law had an OT judicial background that packages a prescribed punishment in the Pentateuch, this punishment prescription is no longer in force. So Catholics are free to adjust the punishment according to modern social realities, prudence, and more fundamental Catholic principles such as freedom, love, and mercy.

About Question 104 Article 3

It's important to distinguish between moral (Question 100), ceremonial (Question 101) and judicial (Questions 104-105) precepts. Moral OT laws are still binding, but not ceremonial and judicial.

Intention to obey is the aspect raised by Article 3 of Question 104. Your quote of "I answer that," is incomplete. If the Sharia Law matches the OT judicial law a Catholic can obey it as long as the intention is to observe Human law, not Divine law. To obey the law as Divine Law is considered a mortal sin because it contradicts faith, the basis for Christian salvation. From Article 3, "I answer that," 2nd paragraph:

... On the other hand, the judicial precepts were not instituted that they might be figures, but that they might shape the state of that people who were directed to Christ. Consequently, when the state of that people changed with the coming of Christ, the judicial precepts lost their binding force: for the Law was a pedagogue, leading men to Christ, as stated in Galatians 3:24. Since, however, these judicial precepts are instituted, not for the purpose of being figures, but for the performance of certain deeds, the observance thereof is not prejudicial to the truth of faith. But the intention of observing them, as though one were bound by the Law, is prejudicial to the truth of faith: because it would follow that the former state of the people still lasts, and that Christ has not yet come.

What a sovereign is allowed to do about OT laws is the focus of the first paragraph of "I answer that". The scenario presumed is a Catholic sovereign, who can impose a Human law similar to the (now defunct) Divine judicial OT law (example: death penalty for adulterer). The same provision for intention applies as per individual above. If the reason to enact the law is to promote Divine Law as if God says those judicial laws are still binding, then the sovereign commits a mortal sin. The sovereign can only enact those laws to promote good government as an environment/inducement so people can exercise their freedom to love better, or for peace in the society by creating a separate court for adherents of other religions.