Guy at my school trying to convert me to Islam. How do i respond?

[Mystic]

"Isn't allah all knowing?"

"Yes."

"Then why does he need to test his followers?  Either he's not all knowing, as you claim, he's evil and gets a kick out of the suffering of his followers, or he doesn't exist."

God knows all things that are possible to know. He will come to know the patient and truthful per Quran, that is because future doesn't exist as of now, and it's impossible to know all of what freewill decisions are going to be made.

This is my particular understanding.

Here are some verses about God coming to know:

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حَسِبْتُمْ أَنْ تَدْخُلُوا الْجَنَّةَ وَلَمَّا يَعْلَمِ اللَّهُ الَّذِينَ جَاهَدُوا مِنْكُمْ وَيَعْلَمَ الصَّابِرِينَ {142}
[Shakir 3:142] Do you think that you will enter the garden while Allah has not yet known those who strive hard from among you, and (He has not) known the patient.
 
وَلَقَدْ فَتَنَّا الَّذِينَ مِنْ قَبْلِهِمْ ۖ فَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ صَدَقُوا وَلَيَعْلَمَنَّ الْكَاذِبِينَ {3}
[Shakir 29:3] And certainly We tried those before them, so Allah will certainly know those who are true and He will certainly know the liars.
 
وَلَيَعْلَمَنَّ اللَّهُ الَّذِينَ آمَنُوا وَلَيَعْلَمَنَّ الْمُنَافِقِينَ {11}
[Shakir 29:11] And most certainly Allah will know those who believe and most certainly He will know the hypocrites.



وَلَنَبْلُوَنَّكُمْ حَتَّىٰ نَعْلَمَ الْمُجَاهِدِينَ مِنْكُمْ وَالصَّابِرِينَ وَنَبْلُوَ أَخْبَارَكُمْ {31}
[Pickthal 47:31] And verily We shall try you till We know those of you who strive hard (for the cause of Allah) and the steadfast, and till We test your record.

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That said, I haven't met anyone holding this view but me.