In the Name of Allah.
The fact that a person died doesn’t necessarily indicate his soul is finally dead. It rather implies the soul leaves the body to continue its existence in the minor or individual hereafter, whether for punishment (or enjoyment). It was reported the Prophet (ﷺ) said: “Whoever dies, his Qiyamah (i.e. Resurrection in the grave) has come unto him.”
Hence, whoever disbelieves in the punishment (and enjoyment) of the grave has rightly not believed and he is not from the people of Sunnah (i.e. Islam). More so, this matter points to some things for us, like; the grave (itself), existence (after death) at Al-Barzakh (which is the interface between this world and the major Hereafter) and the punishment (or enjoyment) in it.
Then, we should know that the single cause of punishment in the grave is evil deeds (i.e. sins). Allah says (in Surah Al-Ghafir [40] verse 46) of Fir’aun and his people (regarding the punishment they are presently experiencing before the major hereafter): “The Fire; they are exposed to it, morning and afternoon, and on the Day when the Hour will be established (it will be said to the angel): Cause the people Fir’aun to enter the severest torment.”
Therefore, let’s be careful of our actions since our existence stops not at this world; we still have the grave (minor Hereafter) and the (major) Hereafter.
I beseech Allah to save us from His Torment. (Aameen)
