How will they believe if they don't change their mind ?
Hi! They won't! It's because having an open heart towards the gospel is absolutely necessary in order to come to repentance that saves. Having an open heart - means to be sincerely seeking God. So many are not sincere in their seeking of God. It makes all the difference between whether they will understand the gospel or not. And after coming to a correct understanding of the gospel - they can then make the decision to repent of their sins and to accept Jesus as Lord (God and Master) and Savior of their life - which will save them.
The Bible talks some about hard hearts and hearts that are open towards God. Everyone's different in that way. But it's absolutely to have some openness towards the gospel - in order for it to be possible for them to be fully open to the gospel and to wholeheartedly receive it.
Here's an example of how hardness of hearts, prevents reception to the gospel:
Matt 13:3-4
3 And he spake many things unto them in parables, saying, Behold, a sower went forth to sow;
4 And when he sowed, some seeds fell by the way side, and the fowls came and devoured them up:
KJV
Matt 13:19
19 When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side.
KJV