It is not Part of Allah's Decree to compel Belief

Allah said:

وَلَوْ شَاء رَبُّكَ ...

And had your Lord willed,

meaning `O Muhammad, if it had been the will of your Lord, He would make all the people of the earth believe in what you have brought to them. But Allah has wisdom in what He does.'

... لآمَنَ مَن فِي الأَرْضِ كُلُّهُمْ جَمِيعًا ...

those on earth would have believed, all of them together.

Similarly, Allah said:

وَلَوْ شَآءَ رَبُّكَ لَجَعَلَ النَّاسَ أُمَّةً وَاحِدَةً وَلاَ يَزَالُونَ مُخْتَلِفِينَ

إِلاَّ مَن رَّحِمَ رَبُّكَ وَلِذلِكَ خَلَقَهُمْ وَتَمَّتْ كَلِمَةُ رَبِّكَ لاّمْلاّنَّ جَهَنَّمَ مِنَ الْجِنَّةِ وَالنَّاسِ أَجْمَعِينَ

And if your Lord had so willed, He could surely have made mankind one Ummah, but they will not cease to disagree. Except him on whom your Lord has bestowed His mercy and for that did He create them. And the Word of your Lord has been fulfilled (His saying): "Surely, I shall fill Hell with Jinn and men all together.'' (11:118-119)

He also said,

أَفَلَمْ يَاْيْـَسِ الَّذِينَ ءَامَنُواْ أَن لَّوْ يَشَآءُ اللَّهُ لَهَدَى النَّاسَ جَمِيعًا

Have not then those who believed yet known that had Allah willed, He could have guided all mankind. (13:31)

Therefore, Allah said:

... أَفَأَنتَ تُكْرِهُ النَّاسَ ...

So, will you then compel mankind,

and force them to believe.

... حَتَّى يَكُونُواْ مُؤْمِنِينَ ﴿٩٩﴾

until they become believers.

meaning, it is not for you to do that. You are not commanded to do that either.

It is Allah Who

يُضِلُّ مَن يَشَآءُ وَيَهْدِى مَن يَشَآءُ

sends astray whom He wills, and guides whom He wills. (35:8)

فَلاَ تَذْهَبْ نَفْسُكَ عَلَيْهِمْ حَسَرَتٍ

So do not destroy yourself in sorrow for them. (35:8)

لَّيْسَ عَلَيْكَ هُدَاهُمْ وَلَـكِنَّ اللَّهَ يَهْدِى مَن يَشَآءُ

It is not up to you to guide them, but Allah guides whom He wills. (2:272)

لَعَلَّكَ بَـخِعٌ نَّفْسَكَ أَلاَّ يَكُونُواْ مُؤْمِنِينَ

It may be that you would kill yourself with grief because they are not believers. (26:3)

إِنَّكَ لاَ تَهْدِى مَنْ أَحْبَبْتَ

you guide not who you like.. (28:56)

فَإِنَّمَا عَلَيْكَ الْبَلَـغُ وَعَلَيْنَا الْحِسَابُ

Your duty is only to convey, and it is up to Us to reckon. (13:40)

فَذَكِّرْ إِنَّمَآ أَنتَ مُذَكِّرٌ

لَّسْتَ عَلَيْهِم بِمُسَيْطِرٍ

So remind, you are only one who reminds. You are not a dictator over them. (88:21-22)

There are other Ayat besides these which prove that Allah is the doer of what He wants, guiding whom He wills, leading whom He wills to stray, all out of His knowledge, wisdom, and justice.