Allaah describes the believers at the beginning of the second surah (chapter) of the Qur'an (translation of the meaning);
So what's the first attribute Allaah describes of the believers? That they believe in the unseen, then after that, that they establish the prayer, and give in charity.
It's believing something even though you don't see it. We believe that Allaah has created angels, that He has sent Noah, Abraham, Moses, Jesus, Muhammad (peace be upon them all and the many other Prophets.) Yet we havn't seen them.
We haven't seen the angels either, but we believe that they exist. The more one believes in Allaah and obeys Him, the more Allaah increases them in faith/emaan. The more faith they have, the more stronger there conviction and belief is in the unseen.
Faith comes along with knowledge, since the person who lacks in knowledge has doubt. Therefore the one who gains knowledge for the sake of Allaah becomes even more aware of the reality. Therefore this increases them in faith.
This is why, there have been many narrations where a pious person was about to leave this temporary life and they felt at ease, they believed the angel of death would approach them so they prepared;
"Verily, those who say: `Our Lord is Allah.' And then stand straight and steadfast, on them the angels will descend, saying: `Fear not, nor grieve! But receive the glad tidings of Paradise which you have been promised!'"
(Quran 41:30)
Whereas those who disbelieve and are rebellious try to run away from the reality of death throughout their lives, but;
Wheresoever you may be, death will overtake you, even if you be in strongly built towers. [Qur'an 4:79]
And when the reality comes;
And if you could but see when the wrongdoers are in the overwhelming pangs of death while the angels extend their hands, [saying], "Discharge your souls! Today you will be awarded the punishment of [extreme] humiliation for what you used to say against Allah other than the truth and [that] you were, toward His verses, being arrogant."
(Quran 6:93)
This person ran away from reality, knowing they would face this some day. Because they rejected Allaah, Allaah will reject them. And they will be punished for their own evils. We seek refuge in Allaah from an evil ending. And may He guide us to what is loved by Him. ameen.