What happens if an act is a sin, but the sin is committed under duress?
First of all, the act remains objectively sinful. An act is a sin based on the three fonts of morality, as they arise from and apply to that choice of the human person. Duress can only affect the subjective aspect of the sin, that is to say, the degree of culpability. Actual sin is an objective sin committed with culpability. And culpability is based on knowledge that the act is objectively immoral, and freedom of the will in choosing the act.
An objective mortal sin is only also an actual mortal sin (which causes the loss of grace and deserves punishment in Hell) if three conditions are met: (1) the act is objectively gravely immoral, (2) the person had full knowledge of the grave immorality of the act at the time of the sin, (3) the act was chosen with full deliberation, that is, with full freedom of will.
However, duress can affect the degree of knowledge or freedom of will, and sometimes make an objective mortal sin not also an actual mortal sin. When under the threat or fear of violence, or some other serious pressure, a fallen sinner may find it difficult to evaluate the morality of an act impartially. And as a result, he may decide that an act is not immoral, or is not gravely immoral, due to duress. His judgment is affected, because he is in the fallen state. And thus, he commits an act mistakenly thinking that it is not a sin or at least not a grave sin.
Duress can also affect the freedom with which the person acts, so that the deliberation is not full. As a result, again, the objective mortal sin would not be also an actual mortal sin. Fear or anxiety over a decision can cause the freedom with which a person acts to be less than full.
Does any degree of duress make an objective sin not also an actual sin? No. A small degree of duress is not sufficient to reduce the culpability of the act substantially. An actual mortal sin is not reduced in culpability to an actual venial sin by a very limited type or degree of duress. In the fallen state, very many sins are committed with some duress. We are under some pressure or influence to commit the sin. That fact alone does not imply that the person is free from culpability or that the act is not an actual sin.
Moreover, an act can still be an actual mortal sin, even if the duress is grave. A Christian who renounces Christ under threat of death or torture might still be guilty with a full culpability. As Christians, we are expected to do good and avoid doing evil, even at great cost.
The evaluation as to whether an act was committed with full knowledge and full freedom of will is a matter for conscience and for the confessional. We can discuss this topic in general terms, but it is a difficult judgment to make, even in one’s own case. Only God can judge our souls.
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