When is stealing a mortal sin?

Last week I took a set of headphones from my gym. I felt guilty and ashamed about it for the following 48 hours so I scraped money together and went to the gym and purchased the headphones. I made it out like it was new purchase and just never grabbed the headphones I paid for. Anyway, I could not go to reconciliation on Saturday and went to Suday’s mass. I was completely prepared to just ask for a blessing when I said “but only say the word and my soul shall be healed” and I begged God to forgive me and I felt it was right for me to take communion after all. NOW I am having second thoughts was I wrapped up in the moment or did God want me in communion? Please help

Baltimore Catechism:

Q. 1291. What sin is it to steal?

A. To steal is a mortal or venial sin, according to the amount stolen either at once or at different times. Circumstances may make the sin greater or less, and they should be explained in confession.

Q. 1301. Are we bound to restore ill-gotten goods?

A. We are bound to restore ill-gotten goods, or the value of them, as far as we are able; otherwise we cannot be forgiven.

When it comes to stealing, the line between mortal and venial sin is one of degree. We have to ask what the impact and damage of the theft was to the victim. Stealing $10 from me is not going to harm me very much, stealing $10 from a homeless person is gravely harmful.
I doubt the gym suffered any grave harm from your theft and you did eventually pay the gym the money for the headphones. In my opinion your sin was venial and you repented and made reparations, therefore you were ok to receive communion. Since this is clearly still bothering your conscience I would definitely recommend that you confess it at your next confession, but I do not believe you sinned mortally.

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