The 6 pillars of Imaan (Faith) is what Muslims MUST believe in. If they don't believe the pillars of Islam/Iman they are not Muslim. Thats why you should always believe in these things.
Believe in Allah (SWT)
You must believe that there is only one God, Allah (SWT). Without associating any partners like mum, dad, statues, idols e.t.c. or you are doing Shirk, the worst sin in Islam which makes you, not a Muslim.
2. Angels belief
You must also believe in Allah (SWT) angels. Believe that they are Allahs (SWT) servants and creations created from Noor (Light). You must accept that Jibra'eel (Gabriel) is the angles leader and that he delivered books to prophets of Allah (SWT).
3. Allahs books
You must believe that Allah (SWT) books were sent down from Jibra'eel to the prophets. You must believe that the Quraan is the one unchanged book of Allah (SWT) and the 4 other books (Tawrah, Injeel, Zaboor, Suhuf lost book) were all true before changed.
4. Allahs Prophets
Believe in Allah (SWT) prophets and that Adam was the first and Muhammad (SAWS) was the last prophet of Allah. Even though only 25 were mentioned in Quraan, you must believe in all of them. For example, Prophet (s) Uzayr, Shameel and Aramaya. You must agree that Allah sent down books to them as well.
5. Yaum-ul-Qiyaam (Judgement day)
You must believe in the day of judgement or Yaum-ul-Qiyaam and that it is on Friday (Jumaah). You must agree that everyone will be resurrected and go to Jannah (Paradise) or Jahannam (Hellfire). You must believe that your book (Of deeds) will be with you and that you will see Allah (SWT).
6. Qadr (Destiny)
Qadr is fate. You must believe that in your book, Allah (SWT) wrote down what happens to you in the future and that he knows everything, like who is going to Jannah and who is successful.
