You mean they are not sure of what actions or inactions in their lives are sinful?
They should examine their conscience. Unless one is a saint, there are many ways in which we are falling short. One might even ask a spouse or a trusted friend, “what are my main faults?” if that can be done without damage to the relationship.
But I really think that with a little reflection, one can discover one’s chief faults. If one has no serious or grave sin, then concentrate on a lesser bad habit that one would like to overcome. Confess that.
I remember a person once saying that he never went to confession because he had nothing to confess. In my mind I thought of the many times he had been unkind to his wife in my presence and of his frequent irritability.
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