With regards to the person’s eemaan decreasing, Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah states:
Then know that the decrease in eemaan is of two types:
The first: the decrease which a person can do nothing about, such as the decrease in the deen of a woman through leaving off the prayer during the days of her menstrual period.
So for certain she has no choice in this. Rather if she wished to offer her prayer (during this time) so that her eemaan would not decrease, it would be said to her:
This is certainly something forbidden for you, and if you were to pray it would surely make your eemaan more deficient!
Therefore this is a deficiency which the person can do nothing about and is not to be blamed for because they have no choice in this matter, without exception.
And for example that a person dies whilst being young. So his eemaan is deficient relative to one whose life is prolonged because for the one whose life is prolonged, his eemaan increases and his actions increase. So he can do nothing about this decrease, so without exception, he is not to be blamed for it.
The second: a decrease through the choice of the person. And this is divided into two types:
If the basis of it is committing a sin or leaving off an obligation, then he is to be blamed for it and he commits a sin through it.
And if his decrease is through leaving off a voluntary act which is not obligatory then he is not to be blamed for it with a blame by which he is deemed to have committed a sin, even though it is upon a person to strive upon righteous action.
(Quoted in Mu’jam at Ta’reefaat p79 to 80, translated by Nasser ibn Najam)