This is a summary of the last part of a book i recommanded here, page 3 comment 10. It's about the (as you can guess from the titles) things that invalidates Islam, That nullifies it.

1. To associate others with Allah in worship [qs:4:116] [qs:5:72] that includes sacrificing for other than Allah, calling on the dead, seeking assistance, taking vows (making promises) in the name of other than Allah. I don’t know about you but back in my younger days (cringe) there was this trend of swearing of your mum’s head. In France, mothers are very sacred, if you want to start a fight, just insult their mum, you’re sure to have a fight. So swearing on your mum’s head meant that you were really very truly truthful. And asking someone to swear on their mum’s head was guarantee of their honesty. Turns out that isn’t a good thing at all...turns out this sounds like shirk..

2. To set up intermediaries between oneself and Allah. And to put your full trust in them. Basically what the Quraysh did. “oh! We’re only using them as intermediaries” [qs:39:3] another example in the Quran here, including Allah’s answer to them: [qs:10:18] k

3. To refuse to accuse the polytheists and those who commit shirk/disbelief or to be in doubt about them being disbelievers, or to approve of their beliefs.

4. To believe that the prophet’s guidance is not complete or perfect; to think that other people’s judgement is better.

5. To hate anything to do with the Prophet (pbuh) and what he came with, even if they act upon it. [qs:47:9]

6. To ridicule anything to do with Islam such as punishment (cutting the hand of thieves) or rewards [qs:9:65-66]

7. Magic in all its form [qs:2:102]

8. Take the disbelievers for friends, support them against the Muslims [qs:5:51]

9. Those who believe they can change or ignore (forsake) the law of Islam [qs:3:85]

10. To turn away from Islam and to stubbornly refuse to learn about its teaching and act upon them. [qs:32:22]

I just want to note that the disbelievers of the time of the prophet (pbuh) were people who KNEW it was the truth and refused to accept it. Like The Quraysh – Abu Jahl and Abu Lahab and all the other Quraysh who persecuted the prophet (pbuh) and his companions when they were living in Macca. They use to put their hands on their ears so as not to hear the ayahs, because they knew deep inside it was the truth.
I was reading the biography of the prophet (pbuh); once the prophet (pbuh) was reciting some Quran and some of the prosecutors were standing nearby, it came to a point where it says to make sujood and they were just so overwhelmed by the surah, they prostrated themselves. Amazing right? I’ll look for the ayah/surah and post it here inshaaAllah.
Something else I’ve learnt recently. The punishment in the grave for learning Quran and not using it/acting upon it and for the one who oversleep at the time of prayer: (i cant find the hadith and the exact wording) but you’ll basically have your head smashed in by an enormous boulder that will then roll away and by the time the man throwing the boulder goes and get it and come back, your head will be back to normal. And that will just keep happening until Judgement day...

May Allah preserve us from ever committing Shirk against His Almighty Self. May He guide us on the straight path and help up deepen our understanding of His beautiful and perfect religion. May we all be protected from the punishment of the grave and may we never fall into the category of the one learning the Quran and not acting upon it. AMEEEEEEEEEEEEENNNNNNNNNNNNN