For example, a person will confess this specific sin but when the time he/she will say it, he/she changes the words to be not very detailed and satisfied already of what is said. Is it witholding a sin?
Well, let’s look at the definition of “mortal sin”. I just googled it on Wikipedia (you could too), and here is what they say:
Its subject matter must be grave.
It must be committed with full knowledge (and awareness) of the sinful action and the gravity of the offense.
It must be committed with deliberate and complete consent.
Does the person who is confessing the sin and who changes the words to “gloss over” or “withhold” some of the sinful detail meet all three (3) criteria for mortal sin?
It is difficult to say. Although in general one should try to be as honest as possible, in some cases an individual might believe (for example) he/she has legitimate reason(s) not to share all the details of a particular event/occurrence with a particular priest.
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