That is the whole idea of muslim faith.To believe in the unseen,i.e.the hell ,the heaven,the angels,the day of judgement and God Himself.The muslims have to believe in them without having any kind of encounter with them.Everyone will believe when they see.Only a believer would believe without seeing.
As in chapter 2:
This is the Book; in it is guidance sure, without doubt, to those who fear Allah;
2:3] Who believe in the Unseen, are steadfast in prayer, and spend out of what We have provided for them;
2:4] And who believe in the Revelation sent to thee, and sent before thy time, and (in their hearts) have the assurance of the Hereafter.
2:5] They are on (true) guidance, from their Lord, and it is these who will prosper
And chapter 102:
102:1] The mutual rivalry for piling up (the good things of this world) diverts you (from the more serious things),
102:2] Until ye visit the graves.
102:3] But nay, ye soon shall know (the reality).
102:4] Again, ye soon shall know!
102:5] Nay, were ye to know with certainty of mind, (ye would beware!)
102:6] Ye shall certainly see Hell-Fire!
102:7] Again, ye shall see it with certainty of sight!
102:8] Then, shall ye be questioned that Day about the joy (ye indulged in!).