We cannot know if a sin is mortal or not?

I’ve heard many times from different clergy and lay Catholics that only God can know if a sin they committed is mortal or not.

Is this true? And what is the citation?

For example…I know it’s a mortal sin to rob a bank. I go ahead and rob a bank anyway after long deliberation…I really cannot know I committed a mortal sin?

For a sin to be mortal it requires three conditions to be satisfied.

Grave matter.
Full knowledge.
Deliberate consent.

Using your example, if you know robbing a bank is a sin of grave matter but you go ahead and do it anyway then assuming you do not lack mental capacity or you are being forced you have committed a mortal sin.

Is this topic a different question than what you asked here?

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I would strongly suggest that you obtain an Examination of Conscience. It is an excellent guide and used properly along with the Sacrament of Reconciliation, can help restore you to a reassuring state of grace. The best I have come across recently is:

A whole buck and a quarter at your local Catholic bookstore.

If you have recurring doubts about the state of your soul, please speak with father. If even that does not help, then consider the possibility that you may suffer from some level of anxiety/OCD/scrupulosity. If so, that is 100% treatable.

If not, just get the examination of conscience.

Obviously since the Church requires us to confees our mortal sins and tells us to form our conscience we then know that we can know if a sin is mortal.

I’ve confessed mortal sin many times and been forgiven. It’s the nature of being a Christian.

Or here

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