One of the necessary requirements for making it to Paradise in the Hereafter is that one needs to believe in the Hereafter.
2:62 Surely, those who believe, those who are Jewish, the Christians, and the converts; anyone who (1) believes in GOD, and (2) believes in the Last Day, and (3) leads a righteous life, will receive their recompense from their Lord. They have nothing to fear, nor will they grieve.
However, for a person to prove that they really believe something, then their actions should correspond with their stated belief; otherwise, the claim is nothing but lip service. For instance, if an investor claims that the market is going to crash, then we would expect that his position in the market would match this belief, but if we see that it doesn’t, then it is a sign that he doesn’t really believe what he states.
At the beginning of the Quran, God informs us of those who claim to believe in the Last Day, yet they do not.
2:8 Then there are those who say, “ We believe in GOD and the Last Day,” while they are not believers.
This shows that simply claiming to believe is not sufficient. Also, God informs us that for our claim to be considered true, it must be tested.
29:2 Do the people think that they will be left to say, “We believe,” without being put to the test?
29:3 We have tested those before them, for GOD must distinguish those who are truthful, and He must expose the liars.
Luckily, God provides three conditions to gauge whether a person genuinely believes in the Hereafter or not.
#1 They are dissatisfied with the mention of God alone and only content when others are mentioned beside God.
39:45 When GOD ALONE is mentioned, the hearts of those who do not believe in the Hereafter shrink with aversion. But when others are mentioned beside Him, they become satisfied.
17:45 When you read the Quran, we place between you and those who do not believe in the Hereafter an invisible barrier. 17:46 We place shields around their minds, to prevent them from understanding it, and deafness in their ears. And when you preach your Lord, using the Quran alone, they run away in aversion.
#3 They accept other source as religious law other than the word of God.
6:112 We have permitted the enemies of every prophet—human and jinn devils—to inspire in each other fancy words, in order to deceive. Had your Lord willed, they would not have done it. You shall disregard them and their fabrications. 6:113 This is to let the minds of those who do not believe in the Hereafter listen to such fabrications, and accept them, and thus expose their real convictions.
This criteria can be used not only for others, but for ourselves as well. If we find that we are failing in one of these regards then perhaps it is an opportunity to reflect, repent, and reform before it is too late.
The Quran further informs us that those who doubt the Hereafter are vulnerable to the devil’s schemes as long as they are in such a state.
34:21 He never had any power over them. But we thus distinguish those who believe in the Hereafter from those who are doubtful about it. Your Lord is in full control of all things.