“Illegal” is not the same thing as “sinful.”
An action is illegal if it contravenes a law established by man, such as the statutes of the Federal, state, or local governments (or whatever juridical order the reader belongs to).
An action is immoral (or objectively sinful) if it contravenes God’s law, which need not coincide in every case with the human law.
Note that, in order for a person actually to commit a sin, he must, evidently, know that what he is doing is wrong (i.e., a contravention of God’s law), and be free to perform that action willingly (e.g., someone who commits a bad action at gunpoint is probably not guilty of a sin).
However, all things being equal, we do have a duty to obey the just laws that are established by our government. That is because, as St. Paul explains, governments receive their authority from God, and when they rule justly, they are actually ruling on behalf of God:
1 Let every person be subject to the governing authorities. For there is no authority except from God, and those that exist have been instituted by God. 2 Therefore whoever resists the authorities resists what God has appointed, and those who resist will incur judgment. 3 For rulers are not a terror to good conduct, but to bad. Would you have no fear of the one who is in authority? Then do what is good, and you will receive his approval, 4 for he is God's servant for your good. But if you do wrong, be afraid, for he does not bear the sword in vain. For he is the servant of God, an avenger who carries out God's wrath on the wrongdoer (Romans 13:1-4, ESV).
However, the submission to authority that St. Paul talks about presupposes that the authority is acting justly. If, on the contrary, a government were to mandate doing something objectively sinful (say, for example, a law that makes it illegal to serve citizens belonging to a certain race), then it would actually be the duty of citizens to disobey the civil law; God’s law is always higher than man’s law. Indeed, an unjust law (such as the one in my example) is not really a law at all. This idea is supported by the following passage in Acts:
And the high priest [a legitimate authority in Jerusalem] questioned them [the apostles], 28 saying, “We strictly charged you not to teach in this name, yet here you have filled Jerusalem with your teaching, and you intend to bring this man's blood upon us.” 29 But Peter and the apostles answered, “We must obey God rather than men” (Acts 5:27-29, ESV).
Note that laws are also unjust if they are excessively burdensome, or if they are impossible to fulfill; for example, a law that declares illegal something that is, in fact, good and holy. In such a case, there is not necessarily a strict duty to disobey this kind of law, but there is also no strict obligation to obey it.
In summary, the relationship between “illegal” and “sinful” can be characterized as follows:
- Any action that contravene’s God’s law is objectively sinful. Only a small fraction of these actions are regulated by human law at all. In other words, many actions are sinful but not illegal as such.
- A just law should in general be obeyed (keeping in mind that no human law is perfect—there might be cases that the legislator did not forsee). Something justly declared illegal in the law would also be sinful.
- An unjust law need not be obeyed, and if the law actually promotes sinful behavior, it actually must be disobeyed. In this case, that which is technically illegal is not sinful, and might even be dutiful.
In all these cases, we are referring to the objective sinfulness of the actions in question; the person acting must also have knowledge and freedom in order to commit an actual sin.