What sins of omission are grave enough to become mortal sins?

[CivisRomanusSum]

We know that omitting to do some things can be sinful. But which omissions are so grave that they may become mortal sins if all the conditions are met?

Also, if neglecting to do the corporal works of mercy is grave, for example, then this expands the scope of inquiry: how often should I do them? If it is grave not to give to the poor, how much should be given? Church teaching is very unspecific on these positive duties so I can’t say for sure whether my omissions are grave.

 

[Fr.Charles_Grondin]

The reason that the Church is not specific in this area is because it is impossible to be specific. When a sin of omission becomes grave is really a matter of proportionality and case by case judgment.

Failing to stop someone from stealing a 50 cent candy bar is wrong but not grave.
Failing to reasonably prevent a murder from taking place is most assuredly a grave omission.

It comes down to whether there was a reasonable opportunity to act to prevent something and whether that thing that we failed to stop was itself a grave matter.

The corporal works of mercy are about caring for our brothers and sisters. Once again, this is a matter of reasonableness and proportionality. The gravity of omitting them would appear to be more in our overall attitude towards our brothers and sisters rather than any one particular act.