Mortal sin is serious sins that cuts us off from sanctifying grace (a share in Gods own blessed life) . If one unrepentantly persists in ANY mortal sin, they damn themselves.
For something to be a mortal sin:
1 It must be of serious matter ( many consider this time be a commiting against one of the ten commandments).
2. It must be done with full knowledge of what you are doing.
3. It must be done with full consent of the will.
To sum it up…it’s a clear/obvious sin that someone commits of their own free will. They are choosing themselves to walk away from God through their word/action/deed/inaction or thought.
One scripture passage that comes to mind that shows not everything is equal:
46 The lord of that servant will come in a day when he looketh not for him, and at an hour when he is not aware, and will cut him in sunder, and will appoint him his portion with the unbelievers.
47 And that servant, which knew his lord’s will, and prepared not himself, neither did according to his will, shall be beaten with many stripes.
48 But he that knew not, and did commit things worthy of stripes, shall be beaten with few stripes. For unto whomsoever much is given, of him shall be much required: and to whom men have committed much, of him they will ask the more.
This is from Luke 12. Here we see a difference in having full knowledge/consent of will and that there is a difference to the Lord.